Inherited nullable field of ObjectType becomes non-nullable in subclass.

I have an ObjectType declared with a nullable field. An ObjectType declared as a subclass of that ObjectType which defines no additional fields generates a schema in which that field is _non_-nullable.

```
@ObjectType('UserType')
export class UserType extends withBaseEntity(BaseUser) {
@Field({nullable: true})
fullName(): string {
return `${this.firstName} ${this.lastName}`
}

@Field(_type => CustomerType, {nullable: true})
customer?: CustomerType

@Field(type => [String], {nullable: 'items'})
roles: string[];

@Field(_type => CustomerType, {nullable: true})
gatekeeperFor?: CustomerType

gatekeeperForId?: string;
}

@ObjectType('ViewerType')
export class ViewerType extends UserType {
}
```
**Expected Behavior**
In the generated schema, I would expect the gatekeeperFor field on ViewerType to be nullable, however it's non-nullable. Note also:

1. The gatekeeperFor field on UserType is correctly generated as nullable.
2. The customer field on both UserType and ViewerType (which is declared in the same way as gatekeeperFor) is correctly generated as nullable.
3. Declaring the gatekeeperFor field in the ViewerType subclass works around this problem.

**Environment (please complete the following information):**

- OS: MacOS 11.3
- Node 14.17.3
- Package version 1.1.1
- TypeScript 4.4.2