HBASE-27878 Fix NPE in balancing rsgroup

[Huginn-kio]

HBASE-27878

Summary

balance_rsgroup can fail with a NullPointerException when an assignment has no current
region server. This change builds the RSGroup balancer input through the common
RegionStates#getAssignmentsForBalancer(...) path.

https://issues.apache.org/jira/browse/HBASE-27878

Root cause

RSGroupInfoManagerImpl#getRSGroupAssignmentsByTable copied entries directly from
RegionStates#getRegionAssignments(). A region with a null current location could
therefore become a null server key, and RSGroupBasedLoadBalancer#correctAssignments
then called getAddress() on that key.

Fix

• Collect online servers in the requested RSGroup and delegate assignment collection to
  RegionStates#getAssignmentsForBalancer(...).
• Reuse the common balancer filtering for null locations, disabled tables, and regions
  that are not OPEN or OPENING, while preserving default-group table membership.
• Add a regression test covering a region with a null current server and preserving
  disabled-table filtering.

Testing

• TestRSGroupsBalance#testGetRSGroupAssignmentsByTable passed with 1 test run, 0 failures, and
  0 errors.
• Neighbor regression slice: TestRSGroupsBalance passed with 4 tests, 0 failures,
  and 0 errors.
• RAT passed with 0 unapproved or unknown licenses.
• SpotBugs passed with 0 bugs and 0 errors.
• Diff-level Checkstyle is clean. A module-wide Checkstyle run reports pre-existing
  violations outside this change.

Compatibility

• Public API impact: none.
• Config impact: none.
• Persisted format impact: none.
• Wire format impact: none.