Show "no owner" when challenge leader cannot be found

Fixes #11768

- Check if `challenge.leader` is `null` before trying to display the user's name after "Created By:"
- Check the `challenge.leader` isn't `null` before trying to get its `id` for comparison when determining if the current user `canDelete` or `canEdit`

website/client/src/components/challenges/challengeItem.vue

@@ -43,7 +43,14 @@
         >
           <div class="owner-item">
             <strong>{{ $t('createdBy') }}:</strong>
+            <span
+              v-if="challenge.leader === null"
+              class="mx-1"
+            >
+              {{ $t('noChallengeOwner') }}
+            </span>
             <user-link
+              v-else
               class="mx-1"
               :user="challenge.leader"
             />