[Timeline] Fix blank traces when impacted_nodes is missing
[Timeline] Fix blank traces when impacted_nodes is missing

The Timeline code assumes that Layout Shift events will always include
impacted_nodes. However, this is not always the case, and when it's
missing the performance timeline fails to load.

This CL updates the logic to handle the data being missing.

Fixes: 1135165
Change-Id: I25b92efa9bae2f65a4d3bd2dd0e5792483369d2e
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2532244
Auto-Submit: Paul Lewis <8915f5d5c39a08c5a3e8a8d7314756e4580816f9@chromium.org>
Reviewed-by: Jan Scheffler <8abd9e3d93041d096604cb2a1b3070dd7075ba7b@chromium.org>
Commit-Queue: Paul Lewis <8915f5d5c39a08c5a3e8a8d7314756e4580816f9@chromium.org>

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