Cleanup of raw nullptr checks with Ref

[AThousandShips]

With optional second commit fully forbidding these comparisons, used to identify these cases, I'd say this should be integrated as we don't really permit checking against nullptr and it'd be good to simply enforce it, though the error messages can be a bit cryptic at times so leaving it open

[AThousandShips]

These cases were redunant

[Hilderin]

That should be very useful!! Everything looks good.

[AThousandShips]

If there's no issues with deleting the operators for future proofing I'll go ahead and squash (I'll keep the operators in godot-cpp for backwards compatibility, as we don't have any DISABLE_DEPRECATED there, and I'll leave a note to not copy it over)

[akien-mga]

I'd agree with deleting the nullptr operators in principle, but this will break compatibility with C++ modules or forks and not just godot-cpp, and in a way that might irritate users, as they might not strive for the same consistency in their codebases.

So I think we need to keep them for compatibility. We can maybe keep them with #ifndef DISABLE_DEPRECATED, so we still have a way to remove them for updating the engine (and this should also be checked by CI, at least on the "Minimal template" build, so not covering the whole codebase).

[AThousandShips]

I'll go ahead and split them off into a separate PR and look at perhaps adding a new define for stricter null-checks or something, as I think DISABLE_DEPRECATED might be too inappropriate for this, and we can work on a good general solution

[akien-mga]

Thanks!

[AThousandShips]

Thank you!