Implement RelationshipSourceCollection for Option<Entity>

[Brezak]

Objective

Option is technically a collection.

Solution

Implement RelationShipSourceCollection for Option<Entity>

Testing

Added a new test

[ItsDoot]

#18087 Decided to go with Entity instead of Option<Entity> for one-to-one relationships, I don't think we need to have two ways to do the same thing.

[alice-i-cecile]

Agreed. I think this is largely redundant, but I'm open to being convinced.

[Brezak]

I've mostly decided to implement RelationshipSourceCollection for every collection that can take it and Option was one of them.

Looking at #18087. I'm mainly concerned about the inevitable situation of a user spawning a entity with the Entity::PLACEHOLDER id and throwing it into a relationship. The RelationShipTarget will end up reporting that it's empty even if it isn't and I don't know what bugs that will cause. The rarity of this situation is going to turn these bugs into the worst kind of heisenbugs. Ones that require the application to be running for an hour before they manifest.

[alice-i-cecile]

Looking at #18087. I'm mainly concerned about the inevitable situation of a user spawning a entity with the Entity::PLACEHOLDER id and throwing it into a relationship

We're restricting the ability to spawn entities with specific IDs and that ID will never be handed out otherwise.

That said, I do sympathize with the desire to implement this trait for every sensible collection type 🤔

[Brezak]

That said, I do sympathize with the desire to implement this trait for every sensible collection type 🤔

That's blocked on #18243 since the remaining collections are implementations of a set (and therefore suffer from the same issue).

I'm going to close this since Entity works and makes this mostly redundant.