[BUG] Unable to clear remote_store.compatibility_mode setting

[BhumikaSaini-Amazon]

Describe the bug

Once set in the cluster settings, there isn't a way to clear the remote_store.compatibility_mode setting

Related component

Storage

To Reproduce

1. Run a docrep cluster to remote-backed cluster migration (steps added in PR https://github.com/opensearch-project/documentation-website/pull/7121/files)

2. Try to clear the compatibility_mode setting:

   PUT "/_cluster/settings?pretty"
   {
       "persistent": {
           "remote_store.compatibility_mode": null
       }
   }

Expected behavior

The setting should get cleared

Additional Details

Error response

{
  "error" : {
    "root_cause" : [
      {
        "type" : "null_pointer_exception",
        "reason" : "Cannot invoke \"String.toLowerCase(java.util.Locale)\" because the return value of \"org.opensearch.common.settings.Settings.get(String)\" is null"
      }
    ],
    "type" : "null_pointer_exception",
    "reason" : "Cannot invoke \"String.toLowerCase(java.util.Locale)\" because the return value of \"org.opensearch.common.settings.Settings.get(String)\" is null"
  },
  "status" : 500
}

Proposed solution
We seem to be missing a null check in validateCompatibilityModeSettingRequest( ):

OpenSearch/server/src/main/java/org/opensearch/action/admin/cluster/settings/TransportClusterUpdateSettingsAction.java

Line 288 in cd55bca

String value = settings.get(RemoteStoreNodeService.REMOTE_STORE_COMPATIBILITY_MODE_SETTING.getKey()).toLowerCase(Locale.ROOT);

Adding that should fix this