Check whether table AM is default

PG 17 added support for DEFAULT in ALTER TABLE .. SET ACCESS METHOD

Relevant PG commit:
d61a6cad6418f643a5773352038d0dfe5d3535b8
postgres/postgres@d61a6ca

In that case, name in AlterTableCmd would be null.
Add a null check here to avoid crash.

(cherry picked from commit 71b9974)
(cherry picked from commit 52d3329)

src/backend/columnar/columnar_tableam.c

@@ -2256,7 +2256,9 @@ ColumnarProcessAlterTable(AlterTableStmt *alterTableStmt, List **columnarOptions
 									"Specify SET ACCESS METHOD before storage parameters, or use separate ALTER TABLE commands.")));
 			}
 
-			destIsColumnar = (strcmp(alterTableCmd->name, COLUMNAR_AM_NAME) == 0);
+			destIsColumnar = (strcmp(alterTableCmd->name ? alterTableCmd->name :
+									 default_table_access_method,
+									 COLUMNAR_AM_NAME) == 0);
 
 			if (srcIsColumnar && !destIsColumnar)
 			{