The TraceObject passed to AppendTraceEvent can be completely
uninitialized (set to all 0s) in some situations where a flush happens
between creation and initialization of the TraceObject.

Fix the MockTraceWriter in test-tracing to expect nullptr strings for
the name of the object so that it does not try to construct an
std::string from a null char*.

Bug: v8:9214
Change-Id: Ib4f3b039ab98d5d786991134cb71ecc62f127179
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1598751
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Auto-Submit: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61265}