Update gapi-chrome-apps.js

JSON.parse("") throws an error, however some Google APIs return a blank response for success.  Ran into this when doing Calendar event delete (https://developers.google.com/google-apps/calendar/v3/reference/events/delete) which returns no response for successful delete.

Propose returning null for such a response.
Merge pull request #239 from hatboysam/patch-1

Update gapi-chrome-apps.js