Don't show "not executable" for files with no coverage information

Summary: We incorrectly merge array() into empty string, which is later interpreted as "this file is entirely not-executable". Instead, show no coverage information in the UI.

Test Plan: Looked at a README diff with no coverage information, got no UI render.

Reviewers: tuomaspelkonen, btrahan, zeeg

Reviewed By: tuomaspelkonen

CC: aran, epriestley

Maniphest Tasks: T965

Differential Revision: https://secure.phabricator.com/D1863

src/unit/result/ArcanistUnitTestResult.php

@@ -89,6 +89,10 @@ final class ArcanistUnitTestResult {
    * @return string Cumulative coverage report.
    */
   public static function mergeCoverage(array $coverage) {
+    if (empty($coverage)) {
+      return null;
+    }
+
     $base = reset($coverage);
     foreach ($coverage as $more_coverage) {
       $len = min(strlen($base), strlen($more_coverage));