[Internal] use node's cluster name as a default for an incoming cluster state who misses it

The ClusterState has a reference to the cluster name since version 1.1.0 (df7474b9fcf849bbfea4222c1d2aa58b6669e52a) . However, if the state was  sent from a master of an older version, this name can be set to null. This is unexpected and can cause bugs. The bad part is that it will never correct itself until a full cluster restart where the cluster state is rebuilt using the code of the latest version.

This commit changes the default to the node's cluster name.

 Relates to #7386