[C#] Enable nullability for variant structs

[Repiteo]

Minor update on the road to GodotSharp being nullable by default. This focuses exclusively on the Variant structs, which are probably the one area that could be shifted over in full without issue. The only non-conversion change was in Projection.cs, where a new null check was added to handle a function returning Vector3? instead of Vector3, matching GDScript implementation.

[raulsntos]

Thanks for helping with the nullable annotations. Looks good, only a small nit-pick.

[raulsntos]

LGTM.

[YuriSizov]

@Repiteo Just a note about contribution style, but we prefer when commit messages start with a verb describing the action in this change. In case of area prefixes the verb would come after them. See this document. This is relevant to your other PRs as well.

[Repiteo]

@YuriSizov Ah! Thanks for clarifying, don't know how that detail escaped me

Based off the documentation, it seems it'd still be acceptable to prefix with C#: , provided it then follows with the verb syntax. Is that indeed the case, or is no prefix preferable?

[AThousandShips]

Here you could do [dotnet] or [C#] or similar to make it clear it's a tag of sorts

My bad got mixed up with the used styles 🙂

[YuriSizov]

Based off the documentation, it seems it'd still be acceptable to prefix with C#: , provided it then follows with the verb syntax. Is that indeed the case, or is no prefix preferable?

Yes, both options are acceptable.

[YuriSizov]

Thanks!