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

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

Author: shahrs87

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

@@ -2593,7 +2593,7 @@ public List<ServerName> getExcludedServersForSystemTable() {
       serverList.add(new Pair<>(s, server.getRegionServerVersion(s)));
     }
     if (serverList.isEmpty()) {
-      return Collections.emptyList();
+      return new ArrayList<>();
     }
     String highestVersion = Collections.max(serverList,
         new Comparator<Pair<ServerName, String>>() {
@@ -2606,7 +2606,7 @@ public int compare(Pair<ServerName, String> o1, Pair<ServerName, String> o2) {
       int comparedValue = VersionInfo.compareVersion(minVersionToMoveSysTables,
           highestVersion);
       if (comparedValue > 0) {
-        return Collections.emptyList();
+        return new ArrayList<>();
       }
     }
     List<ServerName> res = new ArrayList<>();