Fix embarrassing NPE in audit trail.

relates elastic/x-pack-elasticsearch#3988

Original commit: elastic/x-pack-elasticsearch@72c16ea50f

plugin/security/src/main/java/org/elasticsearch/xpack/security/audit/logfile/LoggingAuditTrail.java

@@ -514,7 +514,7 @@ public class LoggingAuditTrail extends AbstractComponent implements AuditTrail,
 
     static Optional<String[]> indices(TransportMessage message) {
         if (message instanceof IndicesRequest) {
-            return Optional.of(((IndicesRequest) message).indices());
+            return Optional.ofNullable(((IndicesRequest) message).indices());
         }
         return Optional.empty();
     }