Allow blank/null on radio.html choices

[Ins1ne]

For this #2559

[tomchristie]

This'd also need including on the vertical and inline layouts.

[jpadilla]

You could also do something like:

{% load i18n %}
{% trans "None" as none_choice %}

and use {{ none_choices }} like on select_multiple.html.

[jpadilla]

This looks good to me as is already.

[tomchristie]

Thanks!