FIX Annot I/O

This PR fixes an issue with `write_labels_to_annot()`. Called with labels from only one hemisphere, it would not write a file for the other hemisphere, so that a simple `write_labels_to_annot(labels, ...); read_labels_from_annot()` would fail (see test in https://github.com/christianbrodbeck/mne-python/commit/a760bf75d3931e3629d64668abd9f3b4b2dc32e2). This PR adds an "unknown" label covering the whole hemisphere to fix that.
Maybe this case should rather just raise an error?