Accept nil state parameter in to_json type signatures

[vpellan]

This PR adds nil state parameter to the type signature for all to_json methods. cState_from_state_s will return an unconfigured instance if nil is passed. A lot of implementations of to_json have nil as the default value.

[ksss]

@vpellan Thank you for your request.
Please tell me the specific use cases that will be improved by this change.
This is a breaking change, so we need information to make a decision.

[vpellan]

Hey @ksss, thanks for your quick reply ! We're currently trying to improve typing in dd-trace-rb, and one of the first step is to re-enable the disabled rules in our Steepfile. When re-enabling Ruby::IncompatibleAssignment, we stumbled on an error on the parameter of a to_json method that we've implemented.
If we change its signature to def to_json: (?JSON::State? state) -> String, a new Ruby::ArgumentTypeMismatch error will appear at line 140 saying that Hash#to_json does not accept nil. Of course we can skip it by changing the parameter to arg = _ = nil or adding a # steep:ignore comment (and even if this PR gets merged, we'll still have to do it until the next release anyway) but I thought that fixing the root cause could be a great overall improvement.

[ksss]

@vpellan
Thank you for the use case.
We plan to discuss it at our next developer meeting.

[ksss]

@vpellan We have decided to accept this PR.
Please wait for RBS v4.
Thanks!