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.

Start at the library entry point that creates Git commits and compare its request construction with the linked GitHub Create a Commit API documentation. Verify that omitting or passing an empty parent produces a root-commit request and that existing parent behavior remains unchanged.

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