Add test with null transform id in stats request (#74130)

Author: przemekwitek

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/transform/action/GetTransformStatsAction.java

@@ -58,7 +58,7 @@ public Request(String id) {
             } else {
                 this.id = id;
             }
-            this.expandedIds = Collections.singletonList(id);
+            this.expandedIds = Collections.singletonList(this.id);
         }
 
         public Request(StreamInput in) throws IOException {

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/transform/action/GetTransformStatsActionRequestTests.java

@@ -15,10 +15,11 @@
 public class GetTransformStatsActionRequestTests extends AbstractWireSerializingTestCase<Request> {
     @Override
     protected Request createTestInstance() {
-        if (randomBoolean()) {
-            return new Request(Metadata.ALL);
-        }
-        return new Request(randomAlphaOfLengthBetween(1, 20));
+        return new Request(
+            randomBoolean()
+                ? randomAlphaOfLengthBetween(1, 20)
+                : randomBoolean() ? Metadata.ALL : null
+        );
     }
 
     @Override