Default required-string User getters to blank strings if unset.

[demiankatz]

While these values are always set in the database, we need these defaults for PrivateUser support.

(This fixes a bug and test failure introduced by #3639).

[demiankatz]

@ThoWagen, I discovered after merging #3639 that it introduced a test failure. I suspect this was not caught sooner because the test in question was introduced close to the time #3639 was opened and may not have been merged to dev yet when I ran tests earlier. In any case, this minor adjustment should get everything under control again -- the issue is that when we store user data in the session instead of the database for privacy mode, there is a chance of null values in unexpected places, and this caused an inappropriate null return from a string-typed method.

[ThoWagen]

I'm wondering if the other getter methods should also return defaults or otherwise throw a custom exception if the value is accessed before being set.

[demiankatz]

@ThoWagen, I had the same thought. Only question is whether to do it here, or to merge this to fix the broken build and then do a more thorough job as a follow-up. Either way, I'll start looking into changes now...

[ThoWagen]

Alright. Then I approve this to quickly fix the bug.

[demiankatz]

@ThoWagen, I built #3658 to create a simple way to call all the getters and see what breaks. That revealed a couple more fields in need of fallbacks, which I have added here.