Distinguish between "no coverage information" and "no test coverage" better

Summary: Fixes T10169. Diffs with no build targets were incorrectly showing as though they had no test coverage, when we actually want to show them having no coverage information available.

Test Plan: Viewed an older revision, saw a column of "Not Executable" before change, now see no column.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10169

Differential Revision: https://secure.phabricator.com/D15044