Prevent undefined response throwing errors during Array.prototype.slice

[tannerhodges]

Hey @Schinkentanz, thanks for putting Daily Git together.

On my first pass, I got this error:

/Users/tannerhodges/.nvm/versions/node/v6.10.3/lib/node_modules/daily-git/src/index.js:95
      var commits = Array.prototype.slice.call(body);
                                          ^

TypeError: Array.prototype.slice called on null or undefined

Some of my repos return empty responses (assuming that's because I either haven't committed anything recently, or because I triggered GitHub's rate limit).

To fix, I added an empty array fallback for cases where body is undefined.