region_mover should gracefully ignore null response from RSGroupAdmin#getRSGroupOfServer

If regionserver has any non-daemon thread running even after it's own shutdown, the running non-daemon thread can prevent clean JVM exit and regionserver could be stuck in the zombie state. We have recently provided a workaround for this in HBASE-26468 for regionserver exit hook to wait 30s for all non-daemon threads to get stopped before terminating JVM abnormally.

However, if regionserver is stuck in such state, region_mover unload fails with:

NoMethodError: undefined method `getName` for nil:NilClass
  getSameRSGroupServers at /bin/region_mover.rb:503
             __ensure__ at /bin/region_mover.rb:313 
          unloadRegions at /bin/region_mover.rb:310               
                 (root) at /bin/region_mover.rb:572               
 

This happens if the cluster has RSGroup enabled and the given server is already stopped, hence RSGroupAdmin#getRSGroupOfServer would return null (as the server is not running anymore so it is not part of any RSGroup). region_mover should ride over this null response and gracefully exit from unloadRegions() call.

We should also check if the fix is applicable to branch-2 and above.