Add with_related_empty methods to EntityWorldMut and EntityCommands

[muddxyii]

Objective

• Improve API for spawning entities with relationships but no additional components.
• Currently this requires awkward syntax: commands.spawn_empty().with_related::<Children>(())
• New syntax: commands.spawn_empty().with_related_empty::<Children>()
• Fixes Allow for adding a relationship without a bundle. #19230

Solution

• Added with_related_empty methods to both EntityWorldMut and EntityCommands

Testing

• Added tests covering both EntityWorldMut and EntityCommands implementations

[alice-i-cecile]

I'm not sold that this is common enough to warrant a helper. @Freyja-moth, can you explain where you ran into this?

[Freyja-moth]

I can't remember exactly where I ran into it (it's quite likely that I've replaced/removed the code), but it is useful for when the relationship has required components and no additional components are needed.

This is admittedly a bit of an arbitrary example, but I'm sure there are similar practical use cases that I just can't think of at the moment.

#[derive(Component)]
#[relationship_target(relationship = Key)]
pub struct HeldKeys(Vec<Entity>);

#[derive(Component)]
#[require(
    Name::new("Key"),
)]
#[relationship(relationship_target = HeldKeys)]
pub struct Key(Entity);

#[derive(Component)]
#[require(Name::new("Special Key")]
#[relationship(relationship_target = SpecialDoor)]
pub struct SpecialKey(pub Entity);

#[derive(Component)]
#[relationship_target(relationship = SpecialKey)]
pub struct SpecialDoor(Entity);

fn spawn_keys(mut commands: Commands, special_door: Single<Entity, With<SpecialDoor>>) {
    commands.spawn(Player)
        .with_related_empty::<HeldKeys>() // Generic key, can be used to open any door, but the special one, since there's no additional components needed beyond the required components it's easier to use the new method.
        .with_related::<HeldKeys>(SpecialKey(*special_door)); // Special key that can only be used to open a door with SpecialDoor
}