ClusterCacheStatsImpl throws exception when cache definition does not exist

The ClusterExecutor is used to execute the DistributedCacheStatsCallable on all nodes in the cluster to retrieve cluster wide stats. However, if one or more of the cluster members are missing a cache definition, then the following exception is thrown ISPN000436: Cache 'someCache' has been requested, but no matching cache configuration exists.

Infinispan's operator Backup/Restore functionality uses zero-capacity pods to join a cluster and perform said operation. On joining the cluster, these nodes do not have any cache definitions defined, resulting in the DistributedCacheStatsCallable failing and cluster-wide cache stats failing until these pods leave the cluster.

A simple solution is to simple update DistributedCacheStatsCallable to return an empty map when executed on a node that is missing a cache configuration.