Log when notify = null during /review

The notify field should be omitted from JSON or set to one of the
supported constants. Log a warning whenever the value is null.
This message may help to identify a potential problem in the Gson
library when parsing input for /review.

Change-Id: I3678c596e9c4884dadf60781fff5352ce8690726

gerrit-server/src/main/java/com/google/gerrit/server/change/PostReview.java

@@ -132,6 +132,7 @@ public class PostReview implements RestModifyView<RevisionResource, Input> {
       checkComments(input.comments);
     }
     if (input.notify == null) {
+      log.warn("notify = null; assuming notify = NONE");
       input.notify = NotifyHandling.NONE;
     }