[profiler][small] Handle empty trace (#58013)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58013
Add a test case and a fix (legacy profiler) for empty trace handling
Test Plan: python test/test_profiler.py
Reviewed By: gdankel
Differential Revision: D28345388
Pulled By: ilia-cher
fbshipit-source-id: 4727589ab83367ac8b506cc0f186e5292d974671
