Add `NOTHING` constant

[nrbnlulu]

I think this is a common usecase

@dataclass
class UpdateSomethingDTO:
	im_optional: Maybe[str] = Nothing()

which would be eaise to do

@dataclass
class UpdateSomethingDTO:
	im_optional: Maybe[str] = NOTHING

There is also this ruff false positive if someone cares about this...