Only set internal error on judging if we have one.

Diskspace errors are not linked to any judging.

(cherry picked from commit ab07c2b)

Author: nickygerritsen

webapp/src/Controller/API/JudgehostController.php

@@ -820,7 +820,9 @@ public function internalErrorAction(Request $request): ?int
         $this->em->persist($error);
         // Even if there are no remaining judge tasks for this judging open (which is covered by the transaction below),
         // we need to mark this judging as internal error.
-        $judging->setInternalError($error);
+        if ($judging) {
+            $judging->setInternalError($error);
+        }
         $this->em->flush();
 
         if ($field_name !== null) {