vmalert: using `notifiers` selector which do not match vmalertmanager

[zekker6]

Currently, using vmalert with misconfigured notifiers which does not match anything leads to creating a vmalert pod with notifiers.url being empty. vmalert does not fail with this configuration (I'll create a separate issue for that), so combined with operator behaviour this leads to successfully running vmalert which will never send any alerts silently.

It would be great to have any sigh of error for such case, e.g. do not create a pod until matching pods of alertmanager do exist or something else to help the user to understand where the issue is.