Fix arg-less TraceEvent.instant()

A copy/paste error inadvertently causes this to be read even when it's
null, leading to a NPE

Bug: None
Change-Id: If5607e7eb13e852427baa9901ae3468701c8157c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5405714


Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279873}