Present codebase does not allow opening root-less .car files

https://github.com/ipld/go-car/blob/v0.0.4/car.go#L120-L122 seems somewhat superfluous. In the tests for https://github.com/ipfs/go-ipfs/pull/7011/ I worked around this by sticking an "empty CID" `\x01\x55\x00\x00` into the root array, but there got to be a better way...?

If there is compelling argument to *not* accepting root-less .car files: please clarify it in this issue.