Fix SourceIdentifier.getRevision()

getRevision() is documented to return null when the revision is not present, but the actual implementation was broken by https://git.opendaylight.org/gerrit/9646 and returns a specialized string.

This breaks at least file generation, as it appends '@0000-00-00' where it should not. Fix this problem and drive users to adjust to null – at least netconf relies on this magic string rather than the API contract.