create_blob: allow creation of empty blobs

A blob with no content is still a valid blob, we should not refuse to
create those. The GitHub API supports them, and so should we.
Add test for empty blob creation
Revert "Add test for empty blob creation"

This reverts commit 266cb8e781594c7d17de856630c103fccbe50d10.
Add integration test to ensure the GitHub API allows for an empty blob
Merge branch 'pr/223' into develop