Clean up code

[bjendres]

Currently, the added code quality control tools make a lot of noise, and we should improve the code accordingly.

However, I would postpone enforcing this until CiviBanking 1.5, so we can get important compatibility released with 1.4.0

[jensschuppe]

That's what the PHPStan baseline should be used for so that checks on new code are reliable (i.e. "come back green"). This issue will then be about gradually fixing errors that are being ignored in the baseline.

[bjendres]

This issue will then be about gradually fixing errors that are being ignored in the baseline.

Yes, that was the intention.
It should also serve as a reminder, as long as we keep a baseline.

[dontub]

The tests pass again: #507 #506 #505 #504.