Fix dashboard activity updates with empty names

What does this MR do?

Fixes the dashboard problems on beta when a wall post was posted from a deleted user.

How confident are you it won't break things if deployed?

Quite sure

How to test

see !1855 (closed)

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• add a short text that can be used in the release notes only on beta
• Once your MR has been merged, you are responsible to create a testing issue in Beta Testing Repo:
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...). how this change can be tested.
  • Be aware, that also non technical people should understand.

Release notes text

(A short text that will appear in the release notes and describes the change for non-technical people. Not always necessary, e.g. not for refactoring.)