disable clay.stats sending when the host or port is configured as null

Right now, if you leave it unconfigured in dev, every call to `stats.send` results in the following traceback:

      File "/Users/jbrown/uber/mupass/env/lib/python2.7/site-packages/clay/stats.py", line 118, in send
        sock.sendto(stat, 0, (self.host, self.port))
    TypeError: coercing to Unicode: need string or buffer, NoneType found