Regression test for #1330 (Coerce None to '')
Possible fix for #1330

Coerce None to '' in CharField.to_native()
CharField - add allow_null argument
Set `allow_none = True` for CharFields with null=True
Test for setting allow_none=True for nullable CharFields
Check the modelfield's class instead
Merge pull request #1348 from yprez/none-to-empty-string_2.4

Coerce None to empty string