fix(proto): correctly serialize FilterExec empty projection (#21885)

## Which issue does this PR close?

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- Closes #21871 

## Rationale for this change

FilterExec supports two semantically different projection states:

- None → return all columns (full projection)
- Some(vec![]) → return no columns (empty projection)
However, both cases were being serialized identically as an empty vector
in the proto representation. During deserialization, an empty vector was
always mapped back to None, meaning an empty projection would silently
become a full projection after a serde round-trip.

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## Are there any user-facing changes?

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Author: Adez017

datafusion/proto/src/physical_plan/mod.rs

@@ -716,18 +716,23 @@ impl protobuf::PhysicalPlanNode {
             })?;
 
         let filter_selectivity = filter.default_filter_selectivity.try_into();
-        let projection = if !filter.projection.is_empty() {
-            Some(
-                filter
-                    .projection
-                    .iter()
-                    .map(|i| *i as usize)
-                    .collect::<Vec<_>>(),
-            )
-        } else {
+        // Preserve the `None` state across proto boundaries. Proto cannot distinguish
+        // between `None` (full projection) and `Some(vec![])` (empty projection) since
+        // both serialize as an empty list. If all columns are included, we reconstruct
+        // `None` to avoid losing this semantic distinction on deserialization.
+        let num_fields = input.schema().fields().len();
+        let mut is_full_projection = filter.projection.len() == num_fields;
+        let mut projection_vec: Vec<usize> = Vec::with_capacity(filter.projection.len());
+        for (i, idx) in filter.projection.iter().enumerate() {
+            let idx = *idx as usize;
+            is_full_projection &= idx == i;
+            projection_vec.push(idx);
+        }
+        let projection = if is_full_projection {
             None
+        } else {
+            Some(projection_vec)
         };
-
         let filter = FilterExecBuilder::new(predicate, input)
             .apply_projection(projection)?
             .with_batch_size(filter.batch_size as usize)
@@ -2339,9 +2344,12 @@ impl protobuf::PhysicalPlanNode {
                             .physical_expr_to_proto(exec.predicate(), codec)?,
                     ),
                     default_filter_selectivity: exec.default_selectivity() as u32,
-                    projection: exec.projection().as_ref().map_or_else(Vec::new, |v| {
-                        v.iter().map(|x| *x as u32).collect::<Vec<u32>>()
-                    }),
+                    projection: match exec.projection() {
+                        None => (0..exec.input().schema().fields().len())
+                            .map(|i| i as u32)
+                            .collect(),
+                        Some(v) => v.iter().map(|x| *x as u32).collect(),
+                    },
                     batch_size: exec.batch_size() as u32,
                     fetch: exec.fetch().map(|f| f as u32),
                 },

datafusion/proto/tests/cases/roundtrip_physical_plan.rs

@@ -3681,3 +3681,40 @@ async fn roundtrip_issue_18602_complex_filter_decode_recursion() -> Result<()> {
 
     roundtrip_test_sql_with_context(sql, &ctx).await
 }
+
+#[test]
+fn roundtrip_filter_with_none_projection() -> Result<()> {
+    let schema = Arc::new(Schema::new(vec![
+        Field::new("a", DataType::Int32, false),
+        Field::new("b", DataType::Int32, false),
+        Field::new("c", DataType::Int32, false),
+    ]));
+    let predicate: Arc<dyn PhysicalExpr> = Arc::new(BinaryExpr::new(
+        Arc::new(Column::new("a", 0)),
+        Operator::Gt,
+        lit(ScalarValue::Int32(Some(0))),
+    ));
+    let input: Arc<dyn ExecutionPlan> = Arc::new(EmptyExec::new(Arc::clone(&schema)));
+
+    // Case 1: None projection (return all columns)
+    roundtrip_test(Arc::new(FilterExec::try_new(
+        Arc::clone(&predicate),
+        Arc::clone(&input),
+    )?))?;
+
+    // Case 2: Some(vec![]) — explicitly empty projection
+    roundtrip_test(Arc::new(
+        FilterExecBuilder::new(Arc::clone(&predicate), Arc::clone(&input))
+            .apply_projection(Some(vec![]))?
+            .build()?,
+    ))?;
+
+    // Case 3: Some(vec![2, 0]) — partial projection
+    roundtrip_test(Arc::new(
+        FilterExecBuilder::new(Arc::clone(&predicate), Arc::clone(&input))
+            .apply_projection(Some(vec![2, 0]))?
+            .build()?,
+    ))?;
+
+    Ok(())
+}