[fix](fe) Fix isCountStar incorrectly treating count(null) as count(*)

### What problem does this PR solve?

Issue Number: close #xxx

Problem Summary: Count.isCountStar() returns true for count(null) because
it only checks `child(0) instanceof Literal` without excluding NullLiteral.
Since count(null) always returns 0 (it counts non-null values of a constant
null), it is semantically different from count(*) which counts all rows.

This could cause incorrect results if downstream rewrite rules (e.g.
PushDownAggThroughJoin, SimplifyWindowExpression, PushCountIntoUnionAll)
use isCountStar() before CountLiteralRewrite has a chance to rewrite
count(null) to 0.

### Release note

Fixed count(null) being incorrectly treated as count(*) in query optimizer,
which could lead to wrong results in certain rewrite scenarios.

### Check List (For Author)

- Test: Unit Test / Regression test
- Behavior changed: No
- Does this need documentation: No

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: yujun777

fe/fe-core/src/main/java/org/apache/doris/nereids/trees/expressions/functions/agg/Count.java

@@ -77,7 +77,7 @@ private Count(boolean isStar, AggregateFunctionParams functionParams) {
     public boolean isCountStar() {
         return isStar
                 || children.isEmpty()
-                || (children.size() == 1 && child(0) instanceof Literal);
+                || (children.size() == 1 && child(0) instanceof Literal && !child(0).isNullLiteral());
     }
 
     @Override

fe/fe-core/src/test/java/org/apache/doris/nereids/rules/rewrite/CountLiteralRewriteTest.java

@@ -18,10 +18,14 @@
 package org.apache.doris.nereids.rules.rewrite;
 
 import org.apache.doris.nereids.trees.expressions.functions.agg.Count;
+import org.apache.doris.nereids.trees.expressions.literal.BigIntLiteral;
+import org.apache.doris.nereids.trees.expressions.literal.NullLiteral;
+import org.apache.doris.nereids.types.BigIntType;
 import org.apache.doris.nereids.util.MemoPatternMatchSupported;
 import org.apache.doris.nereids.util.PlanChecker;
 import org.apache.doris.utframe.TestWithFeService;
 
+import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.Test;
 
 /**
@@ -71,4 +75,32 @@ void testCountNull() {
                 .matches(logicalAggregate().when(agg -> agg.getExpressions().stream().noneMatch(Count.class::isInstance)))
                 .printlnTree();
     }
+
+    @Test
+    void testCountNullIsNotCountStar() {
+        // count(null) should NOT be treated as count(*)
+        Count countNull = new Count(NullLiteral.INSTANCE);
+        Assertions.assertFalse(countNull.isCountStar(),
+                "count(null) should not be count star, because count(null) is always 0");
+
+        // typed null literal: count(CAST(null AS BIGINT)) should NOT be count(*)
+        Count countTypedNull = new Count(new NullLiteral(BigIntType.INSTANCE));
+        Assertions.assertFalse(countTypedNull.isCountStar(),
+                "count(typed null) should not be count star");
+
+        // count(distinct null) should NOT be treated as count(*)
+        Count countDistinctNull = new Count(true, NullLiteral.INSTANCE);
+        Assertions.assertFalse(countDistinctNull.isCountStar(),
+                "count(distinct null) should not be count star");
+
+        // count(1) should be treated as count(*)
+        Count countOne = new Count(new BigIntLiteral(1));
+        Assertions.assertTrue(countOne.isCountStar(),
+                "count(1) should be count star");
+
+        // count(*) should be treated as count(*)
+        Count countStar = new Count();
+        Assertions.assertTrue(countStar.isCountStar(),
+                "count(*) should be count star");
+    }
 }

regression-test/data/nereids_rules_p0/count_null_not_count_star/count_null_not_count_star.out

@@ -0,0 +1,34 @@
+-- This file is automatically generated. You should know what you did if you want to edit this
+-- !count_null --
+0
+
+-- !count_star --
+4
+
+-- !count_literal --
+4
+
+-- !count_null_group_by --
+1	0
+2	0
+3	0
+4	0
+
+-- !count_mixed --
+0	4	2	2
+
+-- !count_null_union --
+0
+
+-- !count_null_window --
+1	0
+2	0
+3	0
+4	0
+
+-- !count_null_join --
+0
+
+-- !count_star_join --
+2	0
+

regression-test/suites/nereids_rules_p0/count_null_not_count_star/count_null_not_count_star.groovy

@@ -0,0 +1,80 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+suite("count_null_not_count_star") {
+    sql "SET enable_nereids_planner=true"
+    sql "SET enable_fallback_to_original_planner=false"
+
+    sql "DROP TABLE IF EXISTS count_null_test"
+
+    sql """
+        create table count_null_test(pk int, a int, b int) distributed by hash(pk) buckets 10
+        properties('replication_num' = '1');
+    """
+
+    sql """
+        insert into count_null_test values(1, 1, 1), (2, null, 2), (3, 3, null), (4, null, null);
+    """
+    sql "sync"
+
+    // count(null) should always return 0 regardless of row count
+    order_qt_count_null """select count(null) from count_null_test"""
+
+    // count(*) should return total number of rows
+    order_qt_count_star """select count(*) from count_null_test"""
+
+    // count(1) should be equivalent to count(*)
+    order_qt_count_literal """select count(1) from count_null_test"""
+
+    // count(null) with group by should return 0 for each group
+    order_qt_count_null_group_by """select pk, count(null) from count_null_test group by pk order by pk"""
+
+    // mixed: count(null) vs count(*) vs count(column)
+    order_qt_count_mixed """select count(null), count(*), count(a), count(b) from count_null_test"""
+
+    // count(null) in subquery with union all
+    order_qt_count_null_union """
+        select count(null) from (
+            select a from count_null_test
+            union all
+            select a from count_null_test
+        ) t
+    """
+
+    // count(null) as window function
+    order_qt_count_null_window """
+        select pk, count(null) over (order by pk) from count_null_test order by pk
+    """
+
+    // count(null) through join must still return 0, not get rewritten by PushDownAggThroughJoin
+    sql "DROP TABLE IF EXISTS count_null_test2"
+    sql """
+        create table count_null_test2(pk int, c int) distributed by hash(pk) buckets 10
+        properties('replication_num' = '1');
+    """
+    sql """insert into count_null_test2 values(1, 10), (2, 20);"""
+    sql "sync"
+
+    order_qt_count_null_join """
+        select count(null) from count_null_test t1 inner join count_null_test2 t2 on t1.pk = t2.pk
+    """
+
+    // count(null) vs count(*) through join for comparison
+    order_qt_count_star_join """
+        select count(*), count(null) from count_null_test t1 inner join count_null_test2 t2 on t1.pk = t2.pk
+    """
+}