[heap] Filter out nullptr when registering externally referenced objects

Ideally they are already filtered on the embedder side. Sometimes
howevever, embedders end up with a Local<T> pointing to a nullptr
object. In this case the best way to filter this is right at the
beginning of the registration process.

BUG=chromium:713667

Review-Url: https://codereview.chromium.org/2836013003
Cr-Commit-Position: refs/heads/master@{#44844}

src/heap/heap.cc

@@ -5662,6 +5662,10 @@ void Heap::TracePossibleWrapper(JSObject* js_object) {
 
 void Heap::RegisterExternallyReferencedObject(Object** object) {
   HeapObject* heap_object = HeapObject::cast(*object);
+  if (heap_object == nullptr) {
+    // We might encounter non-empty handles that point to nullptr.
+    return;
+  }
   DCHECK(Contains(heap_object));
   if (FLAG_incremental_marking_wrappers && incremental_marking()->IsMarking()) {
     IncrementalMarking::MarkGrey(this, heap_object);