Validate xContentType in PutWatchRequest. (#31088)

Trying to post a new watch without any body currently results in a 
NullPointerException. This change fixes that by validating that 
Post and Put requests always have a body.

Closes #30057

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/watcher/transport/actions/put/PutWatchRequest.java

@@ -126,6 +126,9 @@ public ActionRequestValidationException validate() {
         if (source == null) {
             validationException = ValidateActions.addValidationError("watch source is missing", validationException);
         }
+        if (xContentType == null) {
+            validationException = ValidateActions.addValidationError("request body is missing", validationException);
+        }
         return validationException;
     }
 

x-pack/plugin/src/test/resources/rest-api-spec/api/xpack.watcher.put_watch.json

@@ -29,7 +29,7 @@
     },
     "body": {
       "description" : "The watch",
-      "required" : true
+      "required" : false
     }
   }
 }

x-pack/plugin/src/test/resources/rest-api-spec/test/watcher/put_watch/10_basic.yml

@@ -37,3 +37,16 @@
             }
           }
   - match: { _id: "my_watch" }
+
+---
+"Test empty body is rejected by put watch":
+  - do:
+      cluster.health:
+          wait_for_status: yellow
+
+  - do:
+      catch: bad_request
+      xpack.watcher.put_watch:
+        id: "my_watch"
+  - match: { error.root_cause.0.type: "action_request_validation_exception" }
+  - match: { error.root_cause.0.reason: "Validation Failed: 1: request body is missing;" }

x-pack/plugin/watcher/src/test/java/org/elasticsearch/xpack/watcher/transport/action/WatchRequestValidationTests.java

@@ -86,6 +86,12 @@ public void testPutWatchSourceNull() {
         assertThat(e.validationErrors(), hasItem("watch source is missing"));
     }
 
+    public void testPutWatchContentNull() {
+        ActionRequestValidationException e = new PutWatchRequest("foo", BytesArray.EMPTY, null).validate();
+        assertThat(e, is(notNullValue()));
+        assertThat(e.validationErrors(), hasItem("request body is missing"));
+    }
+
     public void testGetWatchInvalidWatchId() {
         ActionRequestValidationException e = new GetWatchRequest("id with whitespaces").validate();
         assertThat(e, is(notNullValue()));