fix: nullable properties indication in Swagger docs

backend/src/api/directors/entities/director.entity.ts

@@ -17,5 +17,5 @@ export class DirectorEntity implements Director {
 
   surname: string;
 
-  birthDate: Date;
+  birthDate: Date | null;
 }

backend/src/api/movies/entities/movie.entity.ts

@@ -19,7 +19,7 @@ export class MovieEntity implements Movie {
 
   intro: string;
 
-  picture: string;
+  picture: string | null;
 
   publishedAt: number;