Return only if changed

Signed-off-by: Git'Fellow <12234510+solracsf@users.noreply.github.com>

core/Migrations/Version28000Date20230803221055.php

@@ -54,9 +54,8 @@ class Version28000Date20230803221055 extends SimpleMigrationStep {
 
 			if (!$table->hasIndex('tp_tasks_uid_appid_ident')) {
 				$table->addIndex(['user_id', 'app_id', 'identifier'], 'tp_tasks_uid_appid_ident');
+				$changed = true;
 			}
-
-			$changed = true;
 		}
 
 		if ($changed) {