Optional properties on inverse side populated as undefined instead of null with LoadStrategy.JOINED

[squivix]

Describe the bug
When fetching with populate and LoadStrategy.JOINED, a null relationship on the inverse side is populated as undefined instead of null even though I have forceUndefined set to false.

Example

User.ts

@Entity()
export class User extends CustomBaseEntity {
    @Property({type: types.string})
    @Unique()
    username!: string;

    @OneToOne({entity: () => Profile, mappedBy: (profile: Profile) => profile.user, nullable: true})
    profile!: Ref<Profile> | null;


    [OptionalProps]?: "profile";
}

Profile.ts

@Entity()
export class Profile extends CustomBaseEntity {
    @OneToOne({entity: () => User, inversedBy: (user) => user.profile, owner: true, hidden: true})
    user!: User;

    @Property({type: types.string})
    name!: string;
}

Session.ts

@Entity()
export class Session extends CustomBaseEntity {
    @Property({type: types.string, length: 255})
    token!: string;

    @ManyToOne({entity: () => User})
    user!: User;
}

Now if we have the following data in the db
users table

id	username
1	user1

session table

id	token	user_id
1	abc123	1

profile table

id	name	user_id

Then we try to fetch Session populating User and its Profile:

const session = await orm.em.findOneOrFail(Session, {token: "abc123"}, {populate: ["user", "user.profile"]});
console.log(session.user.profile); //expected: null, actual: undefined

We get undefined instead of null

Expected behavior
It should be null instead of undefined.

Additional context
This is doesn't seem to happen with LoadStrategy.SELECT_IN. It also doesn't happen when I fetch directly from the entity User, only populating.

Versions

Dependency	Version
node	v18.12.0
typescript	4.6.4
mikro-orm	5.7.13
@mikro-orm/postgresql	5.7.13

[B4nan]

This is rather expected behavior, I will see if it can be fixed somehow, but you are probably getting undefined there because the value is not hydrated at all, not because it is hydrated to undefined - as opposed to using the select-in strategy where the value is hydrated to null which comes from the database.

If you want to speed things up, a complete repro is welcome.