Generate errors for getters that return no value

[garychia]

Replaced #63541
Fixes #63393
Getters now must return a value.

[dalexeev]

This adds validation only for inline getters (PROP_INLINE). Perhaps we should do this for separated getters (PROP_SETGET) as well?

[dalexeev]

Looks good to me, except for a minor note.

Also I noticed that empty return doesn't raise an error/warning:

var property:
    get:
        return

But this is beyond the scope of this PR, as it happens here too:

func my_func() -> Variant:
    return

In my opinion empty return should only be used with void functions or if no return type is specified. In a type is specified or it is getter, return must be non-empty (explicit return null).

[adamscott]

Discussed (a lot) during the GDScript meeting. Currently, we would wait a bit before merging this PR to make sure that a future PR (@dalexeev ?) in that vein (the following citation) would still be compatible with/not make obsolete this PR.

But this is beyond the scope of this PR, as it happens here too:

func my_func() -> Variant:
   return

In my opinion empty return should only be used with void functions or if no return type is specified. In a type is specified or it is getter, return must be non-empty (explicit return null).

There's no rush to merge this PR yet, but thank you @garychia for that PR, we will follow with something soon.