avoiding a NoneType error in method.__doc__

I did a pull request before about this error, but the error reappeared in one of the commits.

This error just happen in our testing server, not in our local machine. The only difference is that the server uses nginx/uwsgi.

It seems method.__doc__ could come empty. and I get this error:

Exception Value: argument of type 'NoneType' is not iterable
Exception Location: .../local/lib/python2.7/site-packages/pyelasticsearch/client.py in _add_es_kwarg_docs, line 40

... and also line 46 now