docs: fix ch 4 so it compiles in strict mode (#40970)

Fixes error: `TS2564: Property 'heroes' has no initializer and is not definitely assigned in the constructor`
and makes `hero` property consistent with ch 3 (ref: #40942)

PR Close #40970

aio/content/examples/toh-pt4/src/app/heroes/heroes.component.ts

@@ -15,10 +15,10 @@ import { MessageService } from '../message.service';
 })
 export class HeroesComponent implements OnInit {
 
-  selectedHero: Hero;
+  selectedHero?: Hero;
 
   // #docregion heroes
-  heroes: Hero[];
+  heroes: Hero[] = [];
   // #enddocregion heroes
 
   constructor(private heroService: HeroService, private messageService: MessageService) { }