Should XLSXListField Support Nullable Values?

[thomasmatecki]

Currently XLSXListField does not behave very nicely if prepping a value of None. This field corresponds to DRF's ListField, which does support allow_null=True.

    def prep_value(self) -> Any:
>       if len(self.value) > 0 and isinstance(self.value[0], Iterable):
E       TypeError: object of type 'NoneType' has no len()

It seems as though this field should nicely handle null (None) values. Happy to open a PR if folks agree.

[rptmat57]

That sounds like a good idea to me. It should definitely handle None and return '' in that case.

[browniebroke]

Fixed in #78