[SPARK-53868][SQL] Use array length check instead of direct reference check in V2ExpressionBuilder

### What changes were proposed in this pull request?

apache#52573 here I did a slight refactor to `visitAggregate`, but I used direct reference check to compare arrays, this works as it's the same reference, but we should make the check more generic to any empty array.

### Why are the changes needed?

If someone were to use different reference in CountStar it would fail, which is not the best behavior as any empty array shuold work.
### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

Existing tests

### Was this patch authored or co-authored using generative AI tooling?

No

Closes apache#52587 from alekjarmov/improve-check.

Authored-by: alekjarmov <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: alekjarmov

sql/catalyst/src/main/java/org/apache/spark/sql/connector/util/V2ExpressionSQLBuilder.java

@@ -287,7 +287,7 @@ protected String visitSQLFunction(String funcName, String[] inputs) {
   protected String visitAggregateFunction(
       String funcName, boolean isDistinct, Expression[] inputs) {
     // CountStar has no children but should return with a star
-    if (funcName.equals("COUNT") && inputs == Expression.EMPTY_EXPRESSION) {
+    if (funcName.equals("COUNT") && inputs.length == 0) {
       return visitAggregateFunction(funcName, isDistinct, new String[]{"*"});
     }
     return visitAggregateFunction(funcName, isDistinct, expressionsToStringArray(inputs));