Synchronization: local doc with null modification date should be updated

The following code in DefaultDocumentDifferencesPolicy does not deal with local documents that have a null modification date. This can be the case if a "partial" synchronization happens (document added but not its properties). This should never happen but some cases were detected.

if (modificationDate == null) {
  log.debug(doc.getPathAsString() + " is skipped because it has no modification date - problem at first import");
}

We should update the doc instead of just logging.