[SPARK-49000][SQL][FOLLOWUP] Improve code style and update comments

### What changes were proposed in this pull request?
Fix `RewriteDistinctAggregates` rule to deal properly with aggregation on DISTINCT literals. Physical plan for `select count(distinct 1) from t`:

```
-- count(distinct 1)
== Physical Plan ==
AdaptiveSparkPlan isFinalPlan=false
+- HashAggregate(keys=[], functions=[count(distinct 1)], output=[count(DISTINCT 1)#2L])
   +- HashAggregate(keys=[], functions=[partial_count(distinct 1)], output=[count#6L])
      +- HashAggregate(keys=[], functions=[], output=[])
         +- Exchange SinglePartition, ENSURE_REQUIREMENTS, [plan_id=20]
            +- HashAggregate(keys=[], functions=[], output=[])
               +- FileScan parquet spark_catalog.default.t[] Batched: false, DataFilters: [], Format: Parquet, Location: InMemoryFileIndex(1 paths)[file:/Users/nikola.mandic/oss-spark/spark-warehouse/org.apache.spark.s..., PartitionFilters: [], PushedFilters: [], ReadSchema: struct<>
```

Problem is happening when `HashAggregate(keys=[], functions=[], output=[])` node yields one row to `partial_count` node, which then captures one row. This four-node structure is constructed by `AggUtils.planAggregateWithOneDistinct`.

To fix the problem, we're adding `Expand` node which will force non-empty grouping expressions in `HashAggregateExec` nodes. This will in turn enable streaming zero rows to parent `partial_count` node, yielding correct final result.

### Why are the changes needed?
Aggregation with DISTINCT literal gives wrong results. For example, when running on empty table `t`:
`select count(distinct 1) from t` returns 1, while the correct result should be 0.
For reference:
`select count(1) from t` returns 0, which is the correct and expected result.

### Does this PR introduce _any_ user-facing change?
Yes, this fixes a critical bug in Spark.

### How was this patch tested?
New e2e SQL tests for aggregates with DISTINCT literals.

### Was this patch authored or co-authored using generative AI tooling?
No.

Closes #47565 from uros-db/SPARK-49000-followup.

Authored-by: Uros Bojanic <157381213+uros-db@users.noreply.github.com>
Signed-off-by: Kent Yao <yao@apache.org>

Author: uros-db

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/RewriteDistinctAggregates.scala

@@ -198,13 +198,15 @@ import org.apache.spark.util.collection.Utils
  */
 object RewriteDistinctAggregates extends Rule[LogicalPlan] {
   private def mustRewrite(
-      aggregateExpressions: Seq[AggregateExpression],
+      distinctAggs: Seq[AggregateExpression],
       groupingExpressions: Seq[Expression]): Boolean = {
-    // If there are any AggregateExpressions with filter, we need to rewrite the query.
-    // Also, if there are no grouping expressions and all aggregate expressions are foldable,
-    // we need to rewrite the query, e.g. SELECT COUNT(DISTINCT 1).
-    aggregateExpressions.exists(_.filter.isDefined) || (groupingExpressions.isEmpty &&
-      aggregateExpressions.exists(_.aggregateFunction.children.forall(_.foldable)))
+    // If there are any distinct AggregateExpressions with filter, we need to rewrite the query.
+    // Also, if there are no grouping expressions and all distinct aggregate expressions are
+    // foldable, we need to rewrite the query, e.g. SELECT COUNT(DISTINCT 1). Without this case,
+    // non-grouping aggregation queries with distinct aggregate expressions will be incorrectly
+    // handled by the aggregation strategy, causing wrong results when working with empty tables.
+    distinctAggs.exists(_.filter.isDefined) || (groupingExpressions.isEmpty &&
+      distinctAggs.exists(_.aggregateFunction.children.forall(_.foldable)))
   }
 
   private def mayNeedtoRewrite(a: Aggregate): Boolean = {
@@ -213,7 +215,6 @@ object RewriteDistinctAggregates extends Rule[LogicalPlan] {
     // We need at least two distinct aggregates or the single distinct aggregate group exists filter
     // clause for this rule because aggregation strategy can handle a single distinct aggregate
     // group without filter clause.
-    // This check can produce false-positives, e.g., SUM(DISTINCT a) & COUNT(DISTINCT a).
     distinctAggs.size > 1 || mustRewrite(distinctAggs, a.groupingExpressions)
   }