Validates an AFF4 type.

def _ValidateAFF4Type(aff4_type):
  """Validates an AFF4 type."""
  if aff4_type is None:
    return

  if not isinstance(aff4_type, type):
    raise TypeError("aff4_type=%s must be a type" % aff4_type)
  if not issubclass(aff4_type, AFF4Object):
    raise TypeError("aff4_type=%s must be a subclass of AFF4Object." %
                    aff4_type)