make warenkorb select required

src/Praktikum/Prak5/Bestellung.php

@@ -127,7 +127,7 @@ HTML;
                     <h2>Warenkorb</h2>
                     <form class="flex-container flex-direction-column nowrap justify-content-center" accept-charset="UTF-8" action="#" method="post">
                         <label for="warenkorb"></label>
-                        <select class="flex-item" id="warenkorb" multiple name="Pizza[]">
+                        <select class="flex-item" id="warenkorb" multiple name="Pizza[]" required>
                         </select>
                     
                         <p id="sum"></p>