…pair
`alert_relabel_configs` in [notifier
config](https://docs.victoriametrics.com/vmalert/#notifier-configuration-file)
can drop alert labels when used to filter different tenant alert message
to different notifier.
alertmanager would report error like `msg="Failed to validate alerts"
err="at least one label pair required"` in this case, but the rest of
the alerts inside one request would still be valid in alertmanager, so
it's not severe.
(cherry picked from commit ae4d376e41)