Lint REST controllers

What does this MR do?

• Annotate parameter and return types in src/Controller/
• Some null checks for session->id()
• Fix a few very minor code issues

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

Should be okay, we might need to change one line if the apps are relying on a certain 401 error code being returned at all times.

Links to related issues

• &6 (closed)

How to test

Reviewing the changes is probably enough, but: call some of the modified Rest endpoints and check they still work

Checklist

• no test, since nothing is supposed to have changed
• 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
• short text that can be used in the release notes forDev
• 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.