Add implicit NULL in CASE WHEN without ELSE clause

CASE expressions without an explicit ELSE clause implicitly return NULL.
Update the explain output to always include this implicit NULL value,
matching ClickHouse's EXPLAIN AST format.

Fixes 4 statements in 4 tests.

Author: claude

internal/explain/functions.go

@@ -836,10 +836,8 @@ func explainCaseExprWithAlias(sb *strings.Builder, n *ast.CaseExpr, alias string
 		}
 	} else {
 		// CASE WHEN ... form
-		argCount := len(n.Whens) * 2
-		if n.Else != nil {
-			argCount++
-		}
+		// CASE without ELSE implicitly has NULL as the else value
+		argCount := len(n.Whens)*2 + 1 // Always add 1 for ELSE (explicit or implicit NULL)
 		if alias != "" {
 			fmt.Fprintf(sb, "%sFunction multiIf (alias %s) (children %d)\n", indent, alias, 1)
 		} else {
@@ -852,6 +850,9 @@ func explainCaseExprWithAlias(sb *strings.Builder, n *ast.CaseExpr, alias string
 		}
 		if n.Else != nil {
 			Node(sb, n.Else, depth+2)
+		} else {
+			// Implicit NULL when no ELSE clause
+			fmt.Fprintf(sb, "%s  Literal NULL\n", indent)
 		}
 	}
 }

parser/testdata/00395_nullable/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt80": true
-  }
-}
+{}

parser/testdata/00688_case_without_else/metadata.json

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

parser/testdata/02346_text_index_function_hasAnyAllTokens/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt319": true
-  }
-}
+{}

parser/testdata/02901_remove_nullable_crash_analyzer/metadata.json

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