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).
I am using the same wording as widespread Nullable/Nonnullable annotations: "nullable" = "may be assigned to null during it's lifetime". As you likely already noticed, the check in question is for more than just boxed primitives. More on that later