Mojo: Fix possible null dereference

There is a branch of Node::OnUserMessageReadAckRequest where the local
`event_to_send` is never initialized from a null state. It is however
unconditionally passed to `ForwardEvent` which assumes a non-null
object.

Bug: None
Change-Id: I3615c02033f58412f2322c7a6df6bad20ab451a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2485847Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#818680}

mojo/core/ports/node.cc

@@ -920,7 +920,8 @@ int Node::OnUserMessageReadAckRequest(
     }
   }
 
-  delegate_->ForwardEvent(peer_node_name, std::move(event_to_send));
+  if (event_to_send)
+    delegate_->ForwardEvent(peer_node_name, std::move(event_to_send));
 
   return OK;
 }