fix(market-card.tsx): add conditional rendering for username to prevent error when data.user_created.username is undefined

src/frontend/src/pages/StorePage/components/market-card.tsx

@@ -171,9 +171,12 @@ export const MarketCardComponent = ({
               </div>
             </CardTitle>
           </div>
-          <span className="text-xs text-primary">
-            by <b>{data.user_created.username}</b>
-          </span>
+          {data.user_created.username && (
+            <span className="text-xs text-primary">
+              by <b>{data.user_created.username}</b>
+            </span>
+          )}
+
           <CardDescription className="pb-2 pt-2">
             <div className="truncate-doubleline">{data.description}</div>
           </CardDescription>