Handle empty seats and rows in theater layout

Updated seat and row data structures to allow null values for seats and states, enabling the display of empty seat spaces and rows. Adjusted rendering logic and styles to visually represent gaps in seating, and enhanced the converter to fill missing seats and rows with placeholders.

src/app/theater-layout/theater-layout.component.ts

@@ -10,7 +10,7 @@ import {TheaterSeatState} from '../model/theater-seat-state.model';
   styleUrl: './theater-layout.component.css'
 })
 export class TheaterLayoutComponent {
-  seatsPerRow = input.required<{ seat: Sitzplatz, state: TheaterSeatState }[][]>();
+  seatsPerRow = input.required<{ seat: Sitzplatz | null, state: TheaterSeatState | null }[][]>();
 
   protected selectedSeatsService = inject(SelectedSeatsService);
 }