userData contains domain without other data

[TimvanSteenbergen]

Incident:
Revisited fsfe.org to find that userid -field was not filled. Cause was userData contains domain without other data. This should be prevented from happening.

Problem
userData gets updates with a record containing only a domain without other data. Should not be allowed

Solution
Add a check to function UserData.persist preventing this.
Also add some check and response on this at function SiteService.persist