ExternalIdentityConflictHandler prone to NPE

ExternalIdentityConflictHandler will throw NPE in case value stored in the REP_LAST_SYNCED property cannot be parsed by ISO8601.parse (and returns null) as shows the following code:

Calendar o = parse(ours.getValue(Type.DATE));
Calendar t = parse(theirs.getValue(Type.DATE));
Calendar v = o.before(t) ? t : o;
parent.setProperty(ours.getName(), v)