@@Servername is NULL

After applying CU8 for SQL 2012 SP2 @@Servername was null 

Somehow ended up in condition of no entry in sys.servers for server_id 0

select * from sys.servers where server_id=0

This returned 0 rows.  Can add the local server with:

sp_addserver '<servername>', local

Then had to restart the server for it to take effect

If it will be a while before can reboot, then suggest using 

serverproperty('ServerName')