Fix NPE raised with ids are null

2017-01-13 15:17:52,641 WARN  [Nuxeo-Work-elasticSearchIndexing-5:1105762330440.1063074036] [Transaction] Unexpected exception from afterCompletion; continuing
java.lang.NullPointerException
	at java.util.Objects.requireNonNull(Objects.java:203)
	at java.lang.String.join(String.java:2501)
	at org.nuxeo.ecm.core.redis.contribs.RedisDBSInvalidations.serializeInvalidations(RedisDBSInvalidations.java:86)
	at org.nuxeo.ecm.core.redis.contribs.RedisDBSInvalidations.serialize(RedisDBSInvalidations.java:79)
	at org.nuxeo.ecm.core.redis.contribs.RedisDBSClusterInvalidator.sendInvalidations(RedisDBSClusterInvalidator.java:212)
	at org.nuxeo.ecm.core.storage.dbs.DBSCachingRepository.sendInvalidationsToOther(DBSCachingRepository.java:284)
	at org.nuxeo.ecm.core.storage.dbs.DBSCachingRepository.commit(DBSCachingRepository.java:146)
	at org.nuxeo.ecm.core.storage.dbs.DBSTransactionState.commit(DBSTransactionState.java:723)
	at org.nuxeo.ecm.core.storage.dbs.DBSSession.commit(DBSSession.java:213)
	at org.nuxeo.ecm.core.storage.dbs.DBSRepositoryBase$TransactionContext.afterCompletion(DBSRepositoryBase.java:418)
	at org.apache.geronimo.transaction.manager.TransactionImpl.afterCompletion(TransactionImpl.java:540)
	at org.apache.geronimo.transaction.manager.TransactionImpl.afterCompletion(TransactionImpl.java:533)
	at org.apache.geronimo.transaction.manager.TransactionImpl.commit(TransactionImpl.java:329)

    private String serializeInvalidations(DBSInvalidations invals) {
        if (invals.all) {
            return ALL_DOCUMENTS;
        }   
        return String.join(ID_SEPARATOR, invals.ids);
    }