`WrappingWebSocketServerFactory` doesn't handle factories that return `None`.

https://twistedmatrix.com/documents/current/api/twisted.internet.interfaces.IProtocolFactory.html#buildProtocol documents that buildProtocol can return None, but when I do that, I get

Traceback (most recent call last):                                                                                                                                                                                 
  File ".../lib/python2.7/site-packages/twisted/protocols/basic.py", line 571, in dataReceived                                                                                                                     
    why = self.lineReceived(line)                                                                                                                                                                                  
  File ".../lib/python2.7/site-packages/twisted/web/http.py", line 1692, in lineReceived                                                                                                                           
    self.allContentReceived()                                                                                                                                                                                      
  File ".../lib/python2.7/site-packages/twisted/web/http.py", line 1781, in allContentReceived                                                                                                                     
    req.requestReceived(command, path, version)                                                                                                                                                                    
  File ".../lib/python2.7/site-packages/twisted/web/http.py", line 768, in requestReceived                                                                                                                         
    self.process()                                                                                                                                                                                                 
--- <exception caught here> ---                                                                                                                                                                                    
  File ".../lib/python2.7/site-packages/twisted/web/server.py", line 183, in process                                                                                                                               
    self.render(resrc)                                                                                                                                                                                             
  File ".../lib/python2.7/site-packages/twisted/web/server.py", line 234, in render                                                                                                                                
    body = resrc.render(self)                                                                                                                                                                                      
  File ".../lib/python2.7/site-packages/autobahn/twisted/resource.py", line 121, in render                                                                                                                         
    protocol = self._factory.buildProtocol(request.transport.getPeer())                                                                                                                                            
  File ".../lib/python2.7/site-packages/autobahn/twisted/websocket.py", line 428, in buildProtocol                                                                                                                 
    proto._proto.transport = proto                                                                                                                                                                                 
exceptions.AttributeError: 'NoneType' object has no attribute 'transport' 

Incidentally, The last line in the traceback should probably be proto._proto.makeConnection(transport).

Start in autobahn/twisted/websocket.py at the buildProtocol implementation shown in the traceback, and compare it with the IProtocolFactory buildProtocol documentation. Reproduce the case where the wrapped factory returns None and verify that the server no longer raises AttributeError; also inspect whether the final connection call should be makeConnection(transport), as reported.