[SPARK-54377][SQL] Fix COMMENT ON TABLE IS NULL to properly remove table comment

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

This PR fixes a bug where COMMENT ON TABLE table_name IS NULL was not properly removing the table comment.

### Why are the changes needed?

The syntax COMMENT ON TABLE table_name IS NULL should remove the table comment. However, the previous implementation was setting the comment to null rather than removing the property entirely.

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

### How was this patch tested?

Enhanced test case test("COMMENT ON TABLE") in DataSourceV2SQLSuite verifies:
* Comment can be set and is stored correctly
* Comment is completely removed when set to NULL (property no longer exists)
* Literal string "NULL" can still be set as a comment value
* Works for both session catalog and V2 catalogs

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

Generated-by: Claude Sonnet 4.5

Closes #53091 from ganeshashree/SPARK-54377.

Authored-by: Ganesha S <ganesha.s@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

Author: ganeshas-db

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/v2AlterTableCommands.scala

@@ -48,7 +48,11 @@ trait AlterTableCommand extends UnaryCommand {
  */
 case class CommentOnTable(table: LogicalPlan, comment: String) extends AlterTableCommand {
   override def changes: Seq[TableChange] = {
-    Seq(TableChange.setProperty(TableCatalog.PROP_COMMENT, comment))
+    if (comment == null) {
+      Seq(TableChange.removeProperty(TableCatalog.PROP_COMMENT))
+    } else {
+      Seq(TableChange.setProperty(TableCatalog.PROP_COMMENT, comment))
+    }
   }
   override protected def withNewChildInternal(newChild: LogicalPlan): LogicalPlan =
     copy(table = newChild)

sql/core/src/test/scala/org/apache/spark/sql/connector/DataSourceV2SQLSuite.scala

@@ -2737,8 +2737,35 @@ class DataSourceV2SQLSuiteV1Filter
     // Session catalog is used.
     withTable("t") {
       sql("CREATE TABLE t(k int) USING json")
+
+      // Verify no comment initially
+      val noCommentRows1 = sql("DESC EXTENDED t").toDF("k", "v", "c")
+        .where("k='Comment'")
+        .collect()
+      assert(noCommentRows1.isEmpty || noCommentRows1.head.getString(1).isEmpty,
+        "Expected no comment initially")
+
+      // Set a comment
       checkTableComment("t", "minor revision")
+
+      // Verify comment is set
+      val commentRows1 = sql("DESC EXTENDED t").toDF("k", "v", "c")
+        .where("k='Comment'")
+        .collect()
+      assert(commentRows1.nonEmpty && commentRows1.head.getString(1) === "minor revision",
+        "Expected comment to be set to 'minor revision'")
+
+      // Remove comment by setting to NULL
       checkTableComment("t", null)
+
+      // Verify comment is removed
+      val removedCommentRows1 = sql("DESC EXTENDED t").toDF("k", "v", "c")
+        .where("k='Comment'")
+        .collect()
+      assert(removedCommentRows1.isEmpty || removedCommentRows1.head.getString(1).isEmpty,
+        "Expected comment to be removed")
+
+      // Set comment to literal "NULL" string
       checkTableComment("t", "NULL")
     }
     val sql1 = "COMMENT ON TABLE abc IS NULL"
@@ -2751,8 +2778,35 @@ class DataSourceV2SQLSuiteV1Filter
     // V2 non-session catalog is used.
     withTable("testcat.ns1.ns2.t") {
       sql("CREATE TABLE testcat.ns1.ns2.t(k int) USING foo")
+
+      // Verify no comment initially
+      val noCommentRows2 = sql("DESC EXTENDED testcat.ns1.ns2.t").toDF("k", "v", "c")
+        .where("k='Comment'")
+        .collect()
+      assert(noCommentRows2.isEmpty || noCommentRows2.head.getString(1).isEmpty,
+        "Expected no comment initially for testcat table")
+
+      // Set a comment
       checkTableComment("testcat.ns1.ns2.t", "minor revision")
+
+      // Verify comment is set
+      val commentRows2 = sql("DESC EXTENDED testcat.ns1.ns2.t").toDF("k", "v", "c")
+        .where("k='Comment'")
+        .collect()
+      assert(commentRows2.nonEmpty && commentRows2.head.getString(1) === "minor revision",
+        "Expected comment to be set to 'minor revision' for testcat table")
+
+      // Remove comment by setting to NULL
       checkTableComment("testcat.ns1.ns2.t", null)
+
+      // Verify comment is removed
+      val removedCommentRows2 = sql("DESC EXTENDED testcat.ns1.ns2.t").toDF("k", "v", "c")
+        .where("k='Comment'")
+        .collect()
+      assert(removedCommentRows2.isEmpty || removedCommentRows2.head.getString(1).isEmpty,
+        "Expected comment to be removed from testcat table")
+
+      // Set comment to literal "NULL" string
       checkTableComment("testcat.ns1.ns2.t", "NULL")
     }
     val sql2 = "COMMENT ON TABLE testcat.abc IS NULL"
@@ -2778,10 +2832,19 @@ class DataSourceV2SQLSuiteV1Filter
 
   private def checkTableComment(tableName: String, comment: String): Unit = {
     sql(s"COMMENT ON TABLE $tableName IS " + Option(comment).map("'" + _ + "'").getOrElse("NULL"))
-    val expectedComment = Option(comment).getOrElse("")
-    assert(sql(s"DESC extended $tableName").toDF("k", "v", "c")
-      .where(s"k='${TableCatalog.PROP_COMMENT.capitalize}'")
-      .head().getString(1) === expectedComment)
+    if (comment == null) {
+      // When comment is NULL, the property should be removed completely
+      val commentRows = sql(s"DESC extended $tableName").toDF("k", "v", "c")
+        .where("k='Comment'")
+        .collect()
+      assert(commentRows.isEmpty || commentRows.head.getString(1).isEmpty,
+        "Expected comment to be removed")
+    } else {
+      val expectedComment = comment
+      assert(sql(s"DESC extended $tableName").toDF("k", "v", "c")
+        .where("k='Comment'")
+        .head().getString(1) === expectedComment)
+    }
   }
 
   test("SPARK-31015: star expression should work for qualified column names for v2 tables") {