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

Both the empty string and `None` evaluate to not true, but `None` will be displayed as "None" in a template. Setting an empty string default obviates the need for template conditionals to display or not display the next value.

This:

```html 
<input type="hidden" name="{{ redirect_field_name }}" value="{{ redirect_field_value }}">
```

Will, with default values, render as this:

```html 
<input type="hidden" name="next" value="None">
```

Which results in a `NoReverseMatch` error because the given URL path is "None". The solution is a template conditional:

```html 
{% if redirect_field_value %}
<input type="hidden" name="{{ redirect_field_name }}" value="{{ redirect_field_value }}">
{% endif %}
```

But this shouldn't be necessary. Since the empty string also evaluate to False this shouldn't be expected to break backwards compatibility for existing code.