fix(validation): accept null for optional union types.

[edwardgeorge]

Validating a field that is an optional union will fail when the value is null:

>>> field = Field(annotation=Optional[Union[int,str]])
>>> field.select_validator()
>>> field.validate(None)
---------------------------------------------------------------------------

molten/validation/field.py in validate(self, value)
    181         if value is None:
    182             if not is_optional:
--> 183                 raise FieldValidationError("this field cannot be null")
    184
    185             return value

FieldValidationError: this field cannot be null

The cause of this is that when determining if a field is optional in
is_optional_annotation the simply checks the second argument of the Union.

This works when the annotation is Optional[X] which expands to
Union[X, NoneType] but not, for instance, with Optional[Union[X, Y]] which
expands to Union[X, Y, NoneType].

We could check the last argument to Union but that would still fail on the,
slightly more contrived, example of Union[Optional[X], Y] which expands to
Union[X, NoneType, Y] and for which null would be perfectly valid.

Therefore, in this implementation, is_optional_annotation is dropped as
that involves iterating over the arguments to determine if one is NoneType
and we also need to iterate over the arguments in extract_optional_annotation
to produce the "inner annotation" so I do both in the same place now instead.
is_optional_annotation is not referenced anywhere else in the project, but
as this affects what is possibly the external API this is possibly a breaking
change.

Adds tests for the examples in test_fields.py.

BREAKING CHANGE: removes is_optional_annotation from molten.typing

[Bogdanp]

Thanks! It looks like this breaks on 3.6 (since typing changed slightly between 3.6 and 3.7) so that has to be addressed. I'll merge it once you do.

[edwardgeorge]

ah, i'll fix it up, thanks.

[Bogdanp]

Hey, @edwardgeorge any update on this? Want me to pick it up?

[edwardgeorge]

Hi, @Bogdanp I've been away from a computer for a couple of weeks whilst moving. feel free to pick up, I probably won't be able to return to it until next week.