don't save null revisions
this should fix https://github.com/remotestorage/remotestorage.js/pull/697#issuecomment-44855755
it's actually not the merge that chokes, but isStaleChild incorrectly
returning false when `node.remote.revision` is falsy, see
https://github.com/remotestorage/remotestorage.js/blob/master/src/sync.js#L16