HBASE-26185 Return mutable list in AssignmentManager#getExcludedServersForSystemTable (apache#3577)

Signed-off-by: Viraj Jasani <vjasani@apache.org>

Author: shahrs87

hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/AssignmentManager.java

@@ -2322,15 +2322,15 @@ public List<ServerName> getExcludedServersForSystemTable() {
       .map(s->new Pair<>(s, master.getRegionServerVersion(s)))
       .collect(Collectors.toList());
     if (serverList.isEmpty()) {
-      return Collections.emptyList();
+      return new ArrayList<>();
     }
     String highestVersion = Collections.max(serverList,
       (o1, o2) -> VersionInfo.compareVersion(o1.getSecond(), o2.getSecond())).getSecond();
     if (!DEFAULT_MIN_VERSION_MOVE_SYS_TABLES_CONFIG.equals(minVersionToMoveSysTables)) {
       int comparedValue = VersionInfo.compareVersion(minVersionToMoveSysTables,
         highestVersion);
       if (comparedValue > 0) {
-        return Collections.emptyList();
+        return new ArrayList<>();
       }
     }
     return serverList.stream()