update behavior of app.result_expires to match celery docs

Celery documentation indicates that app.result_expires of CELERY_RESULT_EXPIRES should respect values of 0 or None: "A value of None or 0 means results will never expire (depending on backend specifications)."

django_celery_results/backends/database.py

@@ -194,6 +194,8 @@ def _forget(self, task_id):
 
     def cleanup(self):
         """Delete expired metadata."""
+        if not self.expires:
+            return
         self.TaskModel._default_manager.delete_expired(self.expires)
         self.GroupModel._default_manager.delete_expired(self.expires)