Fix empty GROUPING SETS output formatting

Output empty grouping set () as 'ExpressionList' without
children count, matching ClickHouse's expected EXPLAIN format.

Fixed tests (10 statements):
- 02293_grouping_function (stmt7)
- 01883_with_grouping_sets (stmt9)
- 02315_grouping_constant_folding (stmt4, stmt6)
- 02416_grouping_function_compatibility (stmt4)
- 03611_uniqExact_bug (stmt10)
- 03708_analyzer_convert_any_outer_to_inner_2 (stmt12)
- 03654_grouping_sets_any_min_max (stmt2, stmt12, stmt14)

Author: claude

internal/explain/select.go

@@ -360,9 +360,14 @@ func explainSelectQuery(sb *strings.Builder, n *ast.SelectQuery, indent string,
 				// but we need to unwrap tuples and output elements directly
 				if lit, ok := g.(*ast.Literal); ok && lit.Type == ast.LiteralTuple {
 					if elements, ok := lit.Value.([]ast.Expression); ok {
-						fmt.Fprintf(sb, "%s  ExpressionList (children %d)\n", indent, len(elements))
-						for _, elem := range elements {
-							Node(sb, elem, depth+3)
+						if len(elements) == 0 {
+							// Empty grouping set () outputs ExpressionList without children count
+							fmt.Fprintf(sb, "%s  ExpressionList\n", indent)
+						} else {
+							fmt.Fprintf(sb, "%s  ExpressionList (children %d)\n", indent, len(elements))
+							for _, elem := range elements {
+								Node(sb, elem, depth+3)
+							}
 						}
 					} else {
 						// Fallback for unexpected tuple value type

parser/testdata/01883_with_grouping_sets/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt9": true
-  }
-}
+{}

parser/testdata/02293_grouping_function/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt7": true
-  }
-}
+{}

parser/testdata/02315_grouping_constant_folding/metadata.json

@@ -1,6 +1 @@
-{
-  "explain_todo": {
-    "stmt4": true,
-    "stmt6": true
-  }
-}
+{}

parser/testdata/02416_grouping_function_compatibility/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt4": true
-  }
-}
+{}

parser/testdata/03611_uniqExact_bug/metadata.json

@@ -1,6 +1,5 @@
 {
   "explain_todo": {
-    "stmt10": true,
     "stmt2": true
   }
 }

parser/testdata/03654_grouping_sets_any_min_max/metadata.json

@@ -1,7 +1 @@
-{
-  "explain_todo": {
-    "stmt12": true,
-    "stmt14": true,
-    "stmt2": true
-  }
-}
+{}

parser/testdata/03708_analyzer_convert_any_outer_to_inner_2/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt12": true
-  }
-}
+{}