Why no rows in sys.dm_os_cluster_* on Availability Group?
This dmv is for Failover Cluster Instance also known as FCI and not for Windows Server Failover Cluster (WSFC).


  These properties are used to set the values that affect failure
  detection, failure response time, and the logging for monitoring the
  health status of the SQL Server failover cluster instance.


Details here:

https://docs.microsoft.com/en-us/sql/relational-databases/system-dynamic-management-views/sys-dm-os-cluster-properties-transact-sql