code: def allow_blank(self, form, name):
          
          if self.blank is not None:
              return self.blank
          model = form._meta.model
          field = model._meta.get_field(self.get_field_name(form, name))
          return field.blank
docs: Allow blank determines if the form might be completely empty. If it's
          empty it will result in a None as the saved value for the ForeignKey.