Gets empty string as default for redirect_field_name

Both the empty string and None evaluate to not true, but None will be
displayed as "None" in a template. This obviates the need for template
conditionals to display or not display the next value.
Merge pull request #49 from bennylope/7c3f886a3c410614e4e7aebb17f2314acb77ccb0

get_context_data key misses return as "None" and possibly rendered in template