sql: fix ALTER TABLE SET LOGGED/UNLOGGED

This was recently added in b614b04, but
for the notice to display correctly we must not return a nil planNode.

No release note since the bug has not been released.

Release note: None

Author: rafiss

pkg/sql/logictest/testdata/logic_test/alter_table

@@ -4563,3 +4563,22 @@ statement ok
 DROP TYPE e1;
 
 subtest end
+
+subtest set_logged
+
+statement ok
+CREATE TABLE t_set_logged (a INT PRIMARY KEY);
+
+skipif config weak-iso-level-configs
+query T noticetrace
+ALTER TABLE t_set_logged SET LOGGED;
+----
+NOTICE: SET LOGGED is not supported and has no effect
+
+skipif config weak-iso-level-configs
+query T noticetrace
+ALTER TABLE t_set_logged SET UNLOGGED;
+----
+NOTICE: SET UNLOGGED is not supported and has no effect
+
+subtest end

pkg/sql/unimplemented.go

@@ -91,5 +91,5 @@ func (p *planner) AlterTableSetLogged(
 			operation,
 		),
 	)
-	return nil, nil
+	return &zeroNode{}, nil
 }