Allow blank/null on radio.html choices.

[aericson]

When field allows null/blank value it is currently impossible to set it to None.

Should check field.allow_null or field.allow_blank and create a blank option as in select.html

[tomchristie]

Yup, fair point. Although it I think it might appear a bit odd. What text vale would we use? none, null, empty, blank?

[aericson]

I guess _('None') would be a good choice:

https://github.com/django/django/blob/1.8a1/django/contrib/admin/options.py#L228

[tomchristie]

Closed by #2631.