Fix server error on email change activation

Closes #966 (closed)

What does this MR do?

Fixes a 500 server error that happens when loading the email-address-change-activation page (page=settings&sub=general&newmail=[token] with a non-existing token, e.g. by reloading the page after a successful activation. The problem was that Database::fetchValueByCriteria throws an exception rather than returning null if the token is not found.

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

Should work

How to test

See issue description

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 (does this need to be included in the release notes?)
• 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.