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

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 an unexpected and can cause bugs. The bad part is that it will never correct it self 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

Closes #7414