Nullability handling improvements, including fixes #31

First two commits were required solely to set up my workflow and make IDE shut up, so feel free to revert them, if you want.

Also includes fix to bug with restoring Character/Byte variables (the existence of bug itself being another proof that _nobody_ uses those nowdays).
> The test with Long is still relevant, doesn't it?

Probably, but old version would fail because of different expected output. Changed it because null checks are supposed to be separate issue from one tested there. I am fine with changing just expected output if you prefer that.