Exception while saving some documents

Because of some yet undetermined causes, sometimes the document archive gets broken, with NULL values in xwikircs. This prevents saving the affected documents, as the archive upgrade is performed in the same transaction as the document save, so if the archive cannot be updated, neither can the document.

Until we find why do these wrong entries get in the database in the first place, a quick workaround needed to have a stable release is to just ignore these wrong fields.