Requesting a branch and getting a 200 response with invalid JSON returns null

[alexanderrtaylor]

Issue:
When requesting data about a single branch via the github-api, if it gets a 404 or nothing back from the branch it will return a null value for the branch rather than throwing an error.

Steps to reproduce: (did this while mocking a github response)

1. Request a single branch by name
2. Fake a returned json for a 200(branch does not exist)
3. Observe that the github-api returns instead a null value

Expected result:
I would normally expect the github API to return a GHFileNotFound error rather than a null value since that is what happens if you make the same request for a tag or PR