_get_FIELD_display throws error if value is None

In the 2.0 release, _get_FIELD_display throws an error if the value is None. I think the code needs to be changed to:

```
def _get_FIELD_display(self, cls):
    value = getattr(cls, self.attname)
    if value is None:
         return value
    return force_text(value.label, strings_only=True)
```
