Extraneous return of empty string in field.export()

[matthewhegarty]

Describe the bug

We have this call in field.export():

    def export(self, instance):
        """
        Returns value from the provided instance converted to export
        representation.
        """
        value = self.get_value(instance)
        if value is None:
            return ""
        return self.widget.render(value, instance)

This means that we have logic which changes the return value within Field. It would be ideal if the widget was responsible for rendering the return value.

This might be a useful modification to v4.