RelationId() on nullable relation returns 'null' string

[angel200sdnot]

Issue type:

[x] bug report
[ ] feature request
[ ] documentation issue

Database system/driver:

[ ] cordova
[ ] mongodb
[ ] mssql
[ ] mysql / mariadb
[ ] oracle
[x] postgres
[ ] cockroachdb
[ ] sqlite
[ ] sqljs
[ ] react-native
[ ] expo

TypeORM version:

[ ] latest
[ ] @next
[x] 0.2.25 (or put your version here)

Steps to reproduce or a small repository showing the problem:

@Entity()
export class ChildEntity {
  @PrimaryGeneratedColumn({ type: 'bigint' })
  id: string;
}

@Entity()
export class ParentEntity {
  @PrimaryGeneratedColumn({ type: 'bigint' })
  id: string;

  @OneToOne(() => ChildEntity, { nullable: true })
  @JoinColumn()
  child: ChildEntity | null;

  @RelationId((parent: ParentEntity) => parent.child)
  childId: string | null;
}

const parent = await entityManager.getRepository(ParentEntity).findOne(parentId);
console.log(parent) // parent.childId will be 'null' if column is null in database

Expected behaviour

childId should be null instead of 'null'

Additional comments

I am getting around this issue by using Column() instead of RelationId()