Don't set the empty string as we display it on the scoreboard

[vmcj]

We state we fallback but would pick the empty string now,

[meisterT]

I seem to remember that we also had an issue with nulls but cannot find it quickly in the code.

[vmcj]

I seem to remember that we also had an issue with nulls but cannot find it quickly in the code.

I do remember the same, but PHPStan seems to be happy so it should be allowed. I think I do prefer null for this specific case or to make the choice explicit here, so if display_name is not set start to guess on something else, the earlier code did not do that so I didn't add it but this feels a bit like having too many fields for the same thing.