Add `NOTHING` constant

I think this is a common usecase
```py
@dataclass
class UpdateSomethingDTO:
	im_optional: Maybe[str] = Nothing()
```
which would be eaise to do
```py
@dataclass
class UpdateSomethingDTO:
	im_optional: Maybe[str] = NOTHING
```

There is also this ruff false positive if someone cares about this...
![image](https://github.com/user-attachments/assets/26404c1a-4592-461d-95a9-7da58441b018)
