Handle timediff's with missing start time.

Fixes #3147

This could happen in race conditions when picking up judgings, or when
upgrading from previous DOMjudge versions.

Author: meisterT

webapp/src/Twig/TwigExtension.php

@@ -185,7 +185,7 @@ public function getGlobals(): array
         ];
     }
 
-    public function printtimediff(float $start, ?float $end = null): string
+    public function printtimediff(?float $start, ?float $end = null): string
     {
         return Utils::printtimediff($start, $end);
     }

webapp/src/Utils/Utils.php

@@ -546,8 +546,11 @@ public static function printtime(string|float|null $datetime, string $format): s
      *
      * Copied from lib/www/print.php
      */
-    public static function printtimediff(float $start, ?float $end = null): string
+    public static function printtimediff(?float $start, ?float $end = null): string
     {
+        if (is_null($start)) {
+            return '-';
+        }
         if (is_null($end)) {
             $end = microtime(true);
         }