fix empty payload

@ssured This is my suggestion version of your PR. Take a look and let me know if it works locally for you, because I still haven't found a good way of running the Ember Data test suite.  Apparently it's possible, but the ED team hasn't generalized it yet.

The only thing I really changed in your PR was style (passes my niggling jshint requirements :stuck_out_tongue:) and other small things, commented below.
Here a `payload !== null` is needed too...