Don't use string 'n/a' where the rest of the code expects a numeric value. Fixes sebastianbergmann#781

Author: dvdoug

src/Report/Html/Renderer/File.php

@@ -243,8 +243,8 @@ private function renderTraitOrClassItems(array $items, Template $template, Templ
                     $item['executablePaths']
                 )->asString();
             } else {
-                $numClasses                      = 'n/a';
-                $numTestedClasses                = 'n/a';
+                $numClasses                      = 0;
+                $numTestedClasses                = 0;
                 $linesExecutedPercentAsString    = 'n/a';
                 $branchesExecutedPercentAsString = 'n/a';
                 $pathsExecutedPercentAsString    = 'n/a';