Fix strlen(null) in DifferentialChangesetViewController loadCoverage()

Summary:
DifferentialChangesetViewController loadCoverage() fails under PHP 8.1 when test coverage is provided for some but not all modified files.

Update the code to replace strlen() with phutil_nonempty_string().

Fixes T15521

Test Plan: View a diff with test coverage for some but not all files.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15521

Differential Revision: https://we.phorge.it/D25328