R1:8cbbc9f96107

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

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