[debug] deal with empty handles when checking callback info.

We may get an empty handle passed to the side-effect check in places
where we have not implemented the flag check yet.

R=luoe@chromium.org

Bug: v8:7515
Change-Id: I088b223c4e8cc3aa262bebe34458c2e95b30e347
Reviewed-on: https://chromium-review.googlesource.com/951768Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51783}

src/api-arguments.cc

@@ -50,6 +50,8 @@ Handle<JSObject> PropertyCallbackArguments::CallIndexedEnumerator(
 
 bool PropertyCallbackArguments::PerformSideEffectCheck(
     Isolate* isolate, Handle<Object> callback_info) {
+  // TODO(7515): always pass a valid callback info object.
+  if (callback_info.is_null()) return false;
   return isolate->debug()->PerformSideEffectCheckForCallback(*callback_info);
 }