Fix NPE when updating settings on managed system indices (elastic#67200)

`expectedValue` can be `null` (and in fact will always be `null` if a key is in `requestedSettings` but not `descriptorSettings`), so using `Objects.equals` prevents an NPE here.

server/src/main/java/org/elasticsearch/action/admin/indices/settings/put/TransportUpdateSettingsAction.java

@@ -19,6 +19,13 @@
 
 package org.elasticsearch.action.admin.indices.settings.put;
 
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.stream.Collectors;
+
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 import org.apache.logging.log4j.message.ParameterizedMessage;
@@ -41,12 +48,6 @@
 import org.elasticsearch.threadpool.ThreadPool;
 import org.elasticsearch.transport.TransportService;
 
-import java.util.ArrayList;
-import java.util.HashMap;
-import java.util.List;
-import java.util.Map;
-import java.util.stream.Collectors;
-
 public class TransportUpdateSettingsAction extends AcknowledgedTransportMasterNodeAction<UpdateSettingsRequest> {
 
     private static final Logger logger = LogManager.getLogger(TransportUpdateSettingsAction.class);
@@ -142,7 +143,7 @@ private Map<String, List<String>> checkForSystemIndexViolations(Index[] concrete
                     final String expectedValue = descriptorSettings.get(key);
                     final String actualValue = requestSettings.get(key);
 
-                    if (expectedValue.equals(actualValue) == false) {
+                    if (Objects.equals(expectedValue, actualValue) == false) {
                         failedKeys.add(key);
                     }
                 }