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}