Cluster Cache Loader returns null as a valid value

Found this at http://stackoverflow.com/questions/27840070/infinispan-cluster-in-invalidation-mode-getkey-returns-null-though-some-node

I tweaked the ClusterCacheLoaderTest to have a 3rd cache manager and I can see intermittent failures in either test due to using a null value from a different node.

Unfortunately this isn't as simple as tweaking ClusteredGetResponseValidityFilter to only say a valid response on a non null, since JGroups will throw an exception when there are no responses.