The field _could_ be used with `null=True`

[aweakley]

Here my code assumes that natural_text will never be None,

python-edtf/edtf/fields.py

Line 138 in fe96fbd

if natural_text == "":

but I think if for some reason you decided to use null=True on your field then it could be None.

[ColeDCrawford]

Somewhat related: I updated a project that was using EDTF v4 to test out the v5 branch and hit this error:

  File "<...>/site-packages/edtf/fields.py", line 123, in update_values
    direct_input = getattr(instance, self.direct_input_field, "")
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: attribute name must be string, not 'NoneType'

This is because I didn't have a direct_input_field defined. Is it reasonable to assume that implementers will have that field, or should we update the fields.py to handle this more gracefully? We should at least call it out in the readme and release notes.

[aweakley]

I've made a PR #68 to add some Django checks for the field.