[7.x][ML] Fix node serialization on GET df-nalytics stats without id (#54808) (#54812)

Previously, the id of the `GetDataFrameAnalyticsStatsAction.Request`
could be `null` which caused NPE on serialization as `writeString`
is used (it doesn't accept null values).

This commit ensures the id is never null.

Closes #54807

Backport of #54808

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/action/GetDataFrameAnalyticsStatsAction.java

@@ -54,7 +54,7 @@ public class GetDataFrameAnalyticsStatsAction extends ActionType<GetDataFrameAna
 
         public static final ParseField ALLOW_NO_MATCH = new ParseField("allow_no_match");
 
-        private String id;
+        private String id = "_all";
         private boolean allowNoMatch = true;
         private PageParams pageParams = PageParams.defaultParams();
 
@@ -94,7 +94,7 @@ public class GetDataFrameAnalyticsStatsAction extends ActionType<GetDataFrameAna
         }
 
         public void setId(String id) {
-            this.id = id;
+            this.id = ExceptionsHelper.requireNonNull(id, DataFrameAnalyticsConfig.ID.getPreferredName());
         }
 
         public String getId() {

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ml/action/GetDataFrameAnalyticsStatsActionRequestTests.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+
+package org.elasticsearch.xpack.core.ml.action;
+
+import org.elasticsearch.test.ESTestCase;
+
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.notNullValue;
+
+public class GetDataFrameAnalyticsStatsActionRequestTests extends ESTestCase {
+
+    public void testRequest_GivenNoId() {
+        GetDataFrameAnalyticsStatsAction.Request request = new GetDataFrameAnalyticsStatsAction.Request();
+        assertThat(request.getId(), notNullValue());
+        assertThat(request.getId(), equalTo("_all"));
+    }
+
+    public void testSetId_GivenNull() {
+        GetDataFrameAnalyticsStatsAction.Request request = new GetDataFrameAnalyticsStatsAction.Request();
+        expectThrows(IllegalArgumentException.class, () -> request.setId(null));
+    }
+
+    public void testSetId_GivenString() {
+        GetDataFrameAnalyticsStatsAction.Request request = new GetDataFrameAnalyticsStatsAction.Request();
+
+        request.setId("foo");
+
+        assertThat(request.getId(), equalTo("foo"));
+    }
+}