Prevent NPE parsing the stop datafeed request. (#33347)

The issue depends on the request parameters being passed in the request
body rather than as query parameters.

Author: davidkyle

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

@@ -79,14 +79,13 @@ public static Request parseRequest(String datafeedId, XContentParser parser) {
         }
 
         private String datafeedId;
-        private String[] resolvedStartedDatafeedIds;
+        private String[] resolvedStartedDatafeedIds = new String[] {};
         private TimeValue stopTimeout = DEFAULT_TIMEOUT;
         private boolean force = false;
         private boolean allowNoDatafeeds = true;
 
         public Request(String datafeedId) {
             this.datafeedId = ExceptionsHelper.requireNonNull(datafeedId, DatafeedConfig.ID.getPreferredName());
-            this.resolvedStartedDatafeedIds = new String[] { datafeedId };
         }
 
         public Request() {

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

@@ -24,6 +24,9 @@ protected Request createTestInstance() {
         if (randomBoolean()) {
             request.setAllowNoDatafeeds(randomBoolean());
         }
+        if (randomBoolean()) {
+            request.setResolvedStartedDatafeedIds(generateRandomStringArray(4, 8, false));
+        }
         return request;
     }