Fix mappings with no original

Fixes issue #2 (together with https://github.com/thlorenz/parse-base64vlq-mappings/pull/1).

Note: The parsed mappings now contain `source: undefined` and `original: undefined`. Perhaps that's not strictly correct, but in practice, only `t.deepEqual()` cares. Most code doesn't distinguish between undefined properties and properties containing `undefined`, so I left them there.