Fix test history page when no commits ran the test
Fix test history page when no commits ran the test

Summary:
The previous logic for longestDuration raised a type error when no non-null
elements were available. This fixes that.

Test Plan: Manual

Reviewers: nate

Reviewed By: nate

Subscribers: changesbot

Projects: #changes_ui

Differential Revision: https://tails.corp.dropbox.com/D197348

dropbox/changes,dropbox/changes,dropbox/changes,dropbox/changes