Fix `userSignatureForNow` sometimes returning nil.

On the iOS simulator, `NSFullUserName()` (and presumably `NSUserName()`) return an empty string; currently we check for `nil`.
This leads to `userSignatureForNow` returning nil and, eventually, a crash. Instead make sure that `NSFullUserName()` and `NSUserName()` don’t return an empty string or `nil`.

Alas I cannot create a test because I've only been able to reproduce on the iOS simulator.