Filter out some Clover coverage to prevent false positives

Summary: Clover reports generated from PHPUnit sometimes give false positives of lines that were not covered by a test that should have actually been not coverable, apparently due to inaccurate static analysis of files that weren't actually executed. This filters coverage reports of files that show no coverage which avoids these false positives. Fixes T10420.

Test Plan:
Looked at coverage reports of files before and after the change

Before:

After:

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, aurelijus

Maniphest Tasks: T10420

Differential Revision: https://secure.phabricator.com/D15343