Avoid unintentional conversion of index to boolean (#208)

Avoid unintentional conversion of index to boolean.

Author: codebymikey

src/Gitonomy/Git/Parser/DiffParser.php

@@ -92,8 +92,8 @@ protected function doParse()
             $oldName = $oldName === '/dev/null' ? null : substr($oldName, 2);
             $newName = $newName === '/dev/null' ? null : substr($newName, 2);
 
-            $oldIndex = $oldIndex !== null ?: '';
-            $newIndex = $newIndex !== null ?: '';
+            $oldIndex = $oldIndex === null ? '' : $oldIndex;
+            $newIndex = $newIndex === null ? '' : $newIndex;
             $oldIndex = preg_match('/^0+$/', $oldIndex) ? null : $oldIndex;
             $newIndex = preg_match('/^0+$/', $newIndex) ? null : $newIndex;
             $file = new File($oldName, $newName, $oldMode, $newMode, $oldIndex, $newIndex, $isBinary);