Fix DiffHelper::calculateFiles() for empty file

Signed-off-by: Jack Cherng <jfcherng@gmail.com>

Author: jfcherng

src/DiffHelper.php

@@ -163,8 +163,9 @@ public static function calculateFiles(
         $newFile = new \SplFileObject($new, 'r');
 
         return static::calculate(
-            $oldFile->fread($oldFile->getSize()),
-            $newFile->fread($newFile->getSize()),
+            // fread() requires the length > 0 hence we plus 1 for empty files
+            $oldFile->fread($oldFile->getSize() + 1),
+            $newFile->fread($newFile->getSize() + 1),
             $renderer,
             $differOptions,
             $rendererOptions