Credits: Fix data model

[ferdnyc]

Following on from #3909, it turns out my use of a defaultdict in the credits model wasn't practical, because the input data file explicitly contains null values, which get converted into None on JSON import. If a dict contains explicit None values, it will return them when those keys are dereferenced, ignoring the type set for the defaultdict.

So, I scrapped the defaultdict, wrote a loop to scrub out any None keys, and protected all of the accesses to person keys by using person.get().

Eliminates tracebacks when viewing credits, and fixes text filtering in the credits views.

[ferdnyc]

Hrrrm, as I feared we are seeing spurious crash-on-exit issues in the unit tests due to PyQtWebEngine (most likely).

The unit tests shouldn't need to load a web backend at all, it's only the spaghetti interdependencies of classes.app, windows.main_window, windows.views.webview, and classes.project_data that forces creation of a web view. Perhaps there's a way to get that under control better.

[ferdnyc]

Well, the CI failures with ubuntu-20.04 are explainable, so I'm going to push ahead and merge this despite them, as credits is pretty broken right now without this fix.