Allow creating a commit without parents

Currently the library always sets a parent, failing GitHub's input validation if giving a falsy parent.

However, [GitHub's commit API](https://developer.github.com/v3/git/commits/#create-a-commit) allows creating commits without parents:

> `parent` The SHAs of the commits that were the parents of this commit. If omitted or empty, the commit will be written as a root commit.