Fix NPE in Rest Layer when GET missing watch.

The REST GET API was trying to render a	null watch on GET which	was causing an NPE. Don't render the watch if it's not found and add a test for this case.

Fixes: elastic/elasticsearch#202

Original commit: elastic/x-pack-elasticsearch@0c8afa63ba

rest-api-spec/test/get_watch/20_missing.yaml

@@ -0,0 +1,14 @@
+---
+"Test get watch api with missing watch":
+  - do:
+      cluster.health:
+          wait_for_status: green
+
+
+  - do:
+      watcher.get_watch:
+        id: "missing_watch"
+        ignore: 404
+
+  - match: { found : false}
+  - match: { _id: "missing_watch" }

src/main/java/org/elasticsearch/watcher/rest/action/RestGetWatchAction.java

@@ -38,9 +38,11 @@ public class RestGetWatchAction extends WatcherRestHandler {
                 builder.startObject()
                         .field("found", response.isFound())
                         .field("_id", response.getId())
-                        .field("_version", response.getVersion())
-                        .field("watch", response.getSource(), ToXContent.EMPTY_PARAMS)
-                        .endObject();
+                        .field("_version", response.getVersion());
+                        if (response.isFound()) {
+                            builder.field("watch", response.getSource(), ToXContent.EMPTY_PARAMS);
+                        }
+                        builder.endObject();
 
                 RestStatus status = response.isFound() ? OK : NOT_FOUND;
                 return new BytesRestResponse(status, builder);