[SPARK-32127][SQL] Check rules for MERGE INTO should use MergeAction.…

…conditition other than MergeAction.children

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

This pr fix a bug of check rules for MERGE INTO.

### Why are the changes needed?

SPARK-30924 adds some check rules for MERGE INTO one of which ensures the first MATCHED clause must have a condition. However, it uses `MergeAction.children` in the checking which is not accurate for the case, and it lets the below case pass the check:

```
MERGE INTO testcat1.ns1.ns2.tbl AS target
xxx
WHEN MATCHED THEN UPDATE SET target.col2 = source.col2
WHEN MATCHED THEN DELETE
xxx
```

We should use `MergeAction.condition` instead.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

An existed ut is modified.

Closes #28943 from xianyinxin/SPARK-32127.

Authored-by: xy_xin <xianyin.xxy@alibaba-inc.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/parser/AstBuilder.scala

@@ -468,7 +468,7 @@ class AstBuilder(conf: SQLConf) extends SqlBaseBaseVisitor[AnyRef] with Logging
       throw new ParseException("There must be at least one WHEN clause in a MERGE statement", ctx)
     }
     // children being empty means that the condition is not set
-    if (matchedActions.length == 2 && matchedActions.head.children.isEmpty) {
+    if (matchedActions.length == 2 && matchedActions.head.condition.isEmpty) {
       throw new ParseException("When there are 2 MATCHED clauses in a MERGE statement, " +
         "the first MATCHED clause must have a condition", ctx)
     }

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/v2Commands.scala

@@ -346,25 +346,25 @@ case class MergeIntoTable(
   override def children: Seq[LogicalPlan] = Seq(targetTable, sourceTable)
 }
 
-sealed abstract class MergeAction(
-    condition: Option[Expression]) extends Expression with Unevaluable {
+sealed abstract class MergeAction extends Expression with Unevaluable {
+  def condition: Option[Expression]
   override def foldable: Boolean = false
   override def nullable: Boolean = false
   override def dataType: DataType = throw new UnresolvedException(this, "nullable")
   override def children: Seq[Expression] = condition.toSeq
 }
 
-case class DeleteAction(condition: Option[Expression]) extends MergeAction(condition)
+case class DeleteAction(condition: Option[Expression]) extends MergeAction
 
 case class UpdateAction(
     condition: Option[Expression],
-    assignments: Seq[Assignment]) extends MergeAction(condition) {
+    assignments: Seq[Assignment]) extends MergeAction {
   override def children: Seq[Expression] = condition.toSeq ++ assignments
 }
 
 case class InsertAction(
     condition: Option[Expression],
-    assignments: Seq[Assignment]) extends MergeAction(condition) {
+    assignments: Seq[Assignment]) extends MergeAction {
   override def children: Seq[Expression] = condition.toSeq ++ assignments
 }
 

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/parser/DDLParserSuite.scala

@@ -1132,8 +1132,8 @@ class DDLParserSuite extends AnalysisTest {
           |MERGE INTO testcat1.ns1.ns2.tbl AS target
           |USING testcat2.ns1.ns2.tbl AS source
           |ON target.col1 = source.col1
-          |WHEN MATCHED THEN DELETE
           |WHEN MATCHED THEN UPDATE SET target.col2 = source.col2
+          |WHEN MATCHED THEN DELETE
           |WHEN NOT MATCHED AND (target.col2='insert')
           |THEN INSERT (target.col1, target.col2) values (source.col1, source.col2)
         """.stripMargin)