`git meta checkout` doesn't work in a repository with no commits

[jhedwardyang]

Repro steps:

git init emptyrepo
cd emptyrepo
git fetch https://github.com/twosigma/git-meta
git meta checkout FETCH_HEAD

Expected:

• Checks out FETCH_HEAD

Actual:

TypeError: Cannot read property 'id' of null
<trace omitted>