Merge remote-tracking branch 'remotes/Nicofuma/ticket/12432' into develop-ascraeus

# By Nicofuma
# Via Nicofuma
* remotes/Nicofuma/ticket/12432:
  [ticket/12432] Correcting the assertion
  [ticket/12432] Adding unit test
  [ticket/12432] Migrator should not revert custom functions

phpBB/phpbb/db/migrator.php

@@ -509,10 +509,17 @@ class migrator
 					throw new \phpbb\db\migration\exception('MIGRATION_INVALID_DATA_CUSTOM_NOT_CALLABLE', $step);
 				}
 
-				return array(
-					$parameters[0],
-					array($last_result),
-				);
+				if ($reverse)
+				{
+					return false;
+				}
+				else
+				{
+					return array(
+						$parameters[0],
+						array($last_result),
+					);
+				}
 			break;
 
 			default: