Ensures :attr:`subject` is either ``None``, or satisfies subsequent (chained) conditions::

            Ensure(None).is_none_or.is_an(int)

def is_none_or(self):
        """
        Ensures :attr:`subject` is either ``None``, or satisfies subsequent (chained) conditions::

            Ensure(None).is_none_or.is_an(int)
        """
        if self._subject is None:
            return NoOpInspector(subject=self._subject, error_factory=self._error_factory)
        else:
            return self