Set default value as not all invocations use 2 arguments

This unbreaks the language versions page. Visiting the page
triggered the functions without the second argument.

Author: vmcj

webapp/src/Twig/TwigExtension.php

@@ -215,7 +215,7 @@ public function printtime(string|float|null $datetime, ?string $format = null, ?
         }
     }
 
-    public function printHumanTimeDiff(float|null $startTime, float|null $endTime): string
+    public function printHumanTimeDiff(float|null $startTime = null, float|null $endTime = null): string
     {
         if ($startTime === null) {
             return '';