Fix typescript warning

Author: J12934

frontend/src/app/deluxe-user/deluxe-user.component.ts

@@ -25,7 +25,7 @@ import { NgIf } from '@angular/common'
 
 export class DeluxeUserComponent implements OnInit {
   public membershipCost: number = 0
-  public error: string = undefined
+  public error?: string = undefined
   public applicationName = 'OWASP Juice Shop'
   public logoSrc: string = 'assets/public/images/JuiceShop_Logo.png'