Fix test regression in snapshot task deletion

Adding an audit callback function with dataset name introduced a
new failure condition for case where snapshot task matching the
id does not exist.

src/middlewared/middlewared/plugins/snapshot.py

@@ -281,8 +281,8 @@ async def do_delete(self, audit_callback, id_, options):
             }
         """
 
-        dataset = (await self.get_instance(id_))['dataset']
-        audit_callback(dataset)
+        if (task := await self.query([['id','=', id_]])):
+            audit_callback(task[0]['dataset'])
 
         for replication_task in await self.middleware.call('replication.query', [
             ['direction', '=', 'PUSH'],