FuzzyDateFormField.__init__ should possibly also pop empty_value from kwargs

[ianmackinnon]

FuzzyDateFormField.__init__ currently pops max_length from kwargs because it may be set as a default by CharField.formfield, but is not valid for forms.MultiValueField.

Under certain circumstances, CharField.formfield also adds a default value for empty_value of None. This is similarly not valid for forms.MultiValueField and so should also probably be popped.

For reference, this is CharField.formfield as it stands in Django 5.1.2:

    def formfield(self, **kwargs):
        # Passing max_length to forms.CharField means that the value's length
        # will be validated twice. This is considered acceptable since we want
        # the value in the form field (to pass into widget for example).
        defaults = {"max_length": self.max_length}
        # TODO: Handle multiple backends with different feature flags.
        if self.null and not connection.features.interprets_empty_strings_as_nulls:
            defaults["empty_value"] = None
        defaults.update(kwargs)
        return super().formfield(**defaults)

I'd be happy to submit a PR with a fix if you'd like.