fix: cardinality() of an empty array should be zero (#20533)

## Which issue does this PR close?

- Closes #20526.

## Rationale for this change

Per Postgres and the SQL spec, `cardinality()` of an empty array should
be zero; we previously returned `NULL`.

Along the way, fix another bug: we previously returned `0` for the
cardinality of an untyped `NULL` and `NULL` for the cardinality of a
typed null (e.g., `NULL::int[]`). We should return `NULL` in both cases.

## What changes are included in this PR?

Bug fixes, update SLT.

## Are these changes tested?

Yes.

## Are there any user-facing changes?

Yes: the behavior of `cardinality` has changed, albeit the previous
behavior was incorrect.

Author: neilconway

datafusion/functions-nested/src/cardinality.rs

@@ -120,7 +120,7 @@ impl ScalarUDFImpl for Cardinality {
 fn cardinality_inner(args: &[ArrayRef]) -> Result<ArrayRef> {
     let [array] = take_function_args("cardinality", args)?;
     match array.data_type() {
-        Null => Ok(Arc::new(UInt64Array::from_value(0, array.len()))),
+        Null => Ok(Arc::new(UInt64Array::new_null(array.len()))),
         List(_) => {
             let list_array = as_list_array(array)?;
             generic_list_cardinality::<i32>(list_array)
@@ -152,9 +152,14 @@ fn generic_list_cardinality<O: OffsetSizeTrait>(
 ) -> Result<ArrayRef> {
     let result = array
         .iter()
-        .map(|arr| match crate::utils::compute_array_dims(arr)? {
-            Some(vector) => Ok(Some(vector.iter().map(|x| x.unwrap()).product::<u64>())),
-            None => Ok(None),
+        .map(|arr| match arr {
+            Some(arr) if arr.is_empty() => Ok(Some(0u64)),
+            arr => match crate::utils::compute_array_dims(arr)? {
+                Some(vector) => {
+                    Ok(Some(vector.iter().map(|x| x.unwrap()).product::<u64>()))
+                }
+                None => Ok(None),
+            },
         })
         .collect::<Result<UInt64Array>>()?;
     Ok(Arc::new(result) as ArrayRef)

datafusion/sqllogictest/test_files/array.slt

@@ -5181,12 +5181,17 @@ select cardinality(arrow_cast([[1, 2], [3, 4], [5, 6]], 'FixedSizeList(3, List(I
 query II
 select cardinality(make_array()), cardinality(make_array(make_array()))
 ----
-NULL 0
+0 0
+
+query II
+select cardinality([]), cardinality([]::int[]) as with_cast
+----
+0 0
 
 query II
 select cardinality(arrow_cast(make_array(), 'LargeList(Int64)')), cardinality(arrow_cast(make_array(make_array()), 'LargeList(List(Int64))'))
 ----
-NULL 0
+0 0
 
 #TODO
 #https://github.com/apache/datafusion/issues/9158
@@ -5195,6 +5200,12 @@ NULL 0
 #----
 #NULL 0
 
+# cardinality of NULL arrays should return NULL
+query II
+select cardinality(NULL), cardinality(arrow_cast(NULL, 'LargeList(Int64)'))
+----
+NULL NULL
+
 # cardinality with columns
 query III
 select cardinality(column1), cardinality(column2), cardinality(column3) from arrays;