ENH: make the mask optional in the nullable masked ExtensionArrays
Our nullable, mask-based ExtensionArrays (currently integer and boolean, inheriting from MaskedArray) have a `_data` and `_mask` numpy arrays stored under the hood.

A relatively straight-forward optimization is probably to allow the `_mask` to be None, for the case that there are no missing values. 
(to be checked how involved the ops code would become to handle this as optional)