Check for null when there are no images

if I did something like "hubot image me ablederfl33wo", I would get an error like:

/opt/hubot/scripts/google-images.coffee:47
      images = images.responseData.results;
                                  ^
TypeError: Cannot read property 'results' of null
    at imageMe (/opt/hubot/scripts/google-images.coffee:47:35)
    at IncomingMessage.ScopedClient.request (/opt/hubot/node_modules/hubot/node_modules/scoped-http-client/lib/index.js:61:20)
    at IncomingMessage.EventEmitter.emit (events.js:126:20)
    at IncomingMessage._emitEnd (http.js:366:10)
    at HTTPParser.parserOnMessageComplete [as onMessageComplete] (http.js:149:23)
    at Socket.socketOnEnd [as onend] (http.js:1356:12)
    at TCP.onread (net.js:418:26)
Merge pull request #370 from danehammer/patch-1

Check for null when there are no images