get_instances() should return empty list instead of Exception

[bbartley]

When a term has no instances, the current behavior is to throw an Exception.

>>> tyto.SO.promoter.get_instances()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/bbartley/Dev/git/Owlet/tyto/tyto.py", line 253, in get_instances
    return self.ontology._handler('get_instances', None, self)
  File "/Users/bbartley/Dev/git/Owlet/tyto/tyto.py", line 72, in _handler
    response = method(self, *args)
  File "/Users/bbartley/Dev/git/Owlet/tyto/endpoint/endpoint.py", line 215, in get_instances
    raise Exception(f'{cls} has no instances')
Exception: https://identifiers.org/SO:0000167 has no instances