Change nonsense return types to Nil: JSON and YAML

[oprypin]

Strictly speaking, this is a backwards-incompatible change, but it should not affect any reasonable code. Attentive review is advised.

First I ran a tool to automatically insert return type restrictions based on what methods actually return in real usages. Then I picked out returns that are obviously purely accidental (due to implicitly returning the last expression) and can never be useful. So I change them to always be Nil.

How to review this:
The 1st commit exists only to provide a more useful diff in the 2nd commit.
Please look only at the 2nd commit (and not the overall diff) to see how this pull request changes actual return types that occur, into Nil.

[straight-shoota]

I suppose the builder methods could even return self to allow method chaining. But that should be a separate change.