notifier: return nil is better. (#5958)
notifier: return nil is better. (#5958)

Signed-off-by: johncming <64aa7c399bffb85fffaccf95be9acb9553c38de5@yahoo.com>
alileza/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,kangwoo/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,keegancsmith/prometheus,kangwoo/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,Gouthamve/prometheus,Gouthamve/prometheus,Gouthamve/prometheus,prometheus/prometheus,alileza/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,prometheus/prometheus,prometheus/prometheus,kangwoo/prometheus,kangwoo/prometheus