[unicode] get_FIELD_display() does not work for None values

[mir@…]

If you have a model field with choices and null=True, the model function get_FIELD_display() returns u'None' if the attribute is None. It should return None

django.db.models.base, lines ~ 322 ff:

    def _get_FIELD_display(self, field):
        value = getattr(self, field.attname)
        return force_unicode(dict(field.choices).get(value, value))

Either this function, or force_unicode, needs a special check for None.

[Malcolm Tredinnick]

We can't make force_unicode() do this by default, because it has to behave like str() in a bunch of places. I'll add a parameter to ignore such non-strings, though, just like smart_str() currently has. Then we can nail them one-by-one (there shouldn't be too many).

[Malcolm Tredinnick]

Change of mind to the previous comment. I cannot see anywhere at all that we are relying on mapping None -> 'None' via force_unicode. So special-casing None seems reasonable.

[Malcolm Tredinnick]

(In [5388]) unicode: Changed handling of None in smart_unicode/force_unicode. There is no
case when converting it to a unicode string seems useful, so keep it as None.
Fixed #4435.

[Malcolm Tredinnick]

(In [5400]) unicode: Reverted [5388] and fixed the problem in a different way. Checked
every occurrence of smart_unicode() and force_unicode() that was not previously
a str() call, so hopefully the problems will not reoccur. Fixed #4447. Refs #4435, #4430.