[inspector] should ignore asyncTask* with null

In V8Debugger code we don't expect task_id == null, e.g.
asyncTaskStartedForStepping will trigger debug break on null as task_id.
Let's filter task_id == null out.

This issue is originally filed in Node.js:
https://github.com/nodejs/node/issues/15464

R=dgozman@chromium.org

Bug: none
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Icc9f96105b3c91ee1b102d545a7817f7ee93394c
Reviewed-on: https://chromium-review.googlesource.com/695808Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48265}

src/inspector/v8-inspector-impl.cc

@@ -289,18 +289,22 @@ std::unique_ptr<V8StackTrace> V8InspectorImpl::captureStackTrace(
 
 void V8InspectorImpl::asyncTaskScheduled(const StringView& taskName, void* task,
                                          bool recurring) {
+  if (!task) return;
   m_debugger->asyncTaskScheduled(taskName, task, recurring);
 }
 
 void V8InspectorImpl::asyncTaskCanceled(void* task) {
+  if (!task) return;
   m_debugger->asyncTaskCanceled(task);
 }
 
 void V8InspectorImpl::asyncTaskStarted(void* task) {
+  if (!task) return;
   m_debugger->asyncTaskStarted(task);
 }
 
 void V8InspectorImpl::asyncTaskFinished(void* task) {
+  if (!task) return;
   m_debugger->asyncTaskFinished(task);
 }