code: def _validate_nullable(self, nullable, field, value):
          
          if value is None:
              if not nullable:
                  self._error(field, errors.NOT_NULLABLE)
              self._drop_remaining_rules(
                  'allowed',
                  'empty',
                  'forbidden',
                  'items',
                  'keysrules',
                  'min',
                  'max',
                  'minlength',
                  'maxlength',
                  'regex',
                  'schema',
                  'type',
                  'valuesrules',
              )
docs: {'type': 'boolean'}