NPE caused by converting uppercase hostname to lowercase in RegionMover

In HBASE-19456, we introduced case-insensitivity feature in RegionMover and converted uppercase hostnames to lowercase hostnames. But this maybe causes that we can't get the rsgroup info of unloading server, because the addresses in hbase are case insensitive. This will make org.apache.hadoop.hbase.util.TestRegionMoverWithRSGroupEnable fail.
 
 
2022-01-27T20:53:31,948 INFO [Time-limited test] util.TestRegionMoverWithRSGroupEnable(127): Unloading VM-154-75-centos
2022-01-27T20:53:31,959 INFO [RpcServer.default.FPBQ.Fifo.handler=2,queue=0,port=49232] master.MasterRpcServices(3011): rsGroupInfo of vm-154-75-centos:39126 is null
2022-01-27T20:53:31,961 INFO [pool-332-thread-1] util.RegionMover(419): rsgroup of vm-154-75-centos:39126 is null
2022-01-27T20:53:31,961 ERROR [pool-332-thread-1] util.RegionMover(471): Error while unloading regions
java.lang.NullPointerException: null
at org.apache.hadoop.hbase.util.RegionMover.lambda$unloadRegions$3(RegionMover.java:421) ~[classes/:?]
at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_292]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_292]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_292]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_292]