Check for userId null value in ConfigService

[arrys]

• Resolves: Server error when accessing Nextcloud Deck on browser "cold" start #3248
• Target version: master

Summary

This is not really a fix, merely a quick patch to prevent the error page from showing.
It does not fix the underlying issue in any way.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Documentation (manuals or wiki) has been updated or is not required

[juliusknorr]

Thanks a lot for your pull request. I've started working on a more wide fix for the underlying issue in #3263 but will also extend that to apply a null check as you proposed, which definitiely also makes sense as an addition. I'll close this in favor of #3263