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).
You could move this to the IcepickProcessorTest, doesn't need to be a separate test.
Also, could you please follow the same style as the other tests? E.g. using `Joiner.on('\n') ` instead of hardcoding newlines?