Revisit `YoleckEntityRef`'s internal optionality

[idanarye]

Both of YoleckEntityRef's fields are Option:

bevy-yoleck/src/entity_ref.rs

Lines 57 to 63 in 40ccb74

	#[derive(Clone, PartialEq, Eq, Hash, Serialize, Deserialize, Default, Debug)]
	pub struct YoleckEntityRef {
	#[serde(default, skip_serializing_if = "Option::is_none")]
	uuid: Option<Uuid>,
	#[serde(skip)]
	resolved: Option<Entity>,
	}

resolved has to be optional, because otherwise we won't be able to deserialize it. But theoretically the uuid should not be optional and instead the YoleckEntityRef can be put inside an Option - because when the uuid is None the resolved is meaningless.

Then again - even though it'd be more correct that way, usability-wise the YoleckEntityRef will have to always be inside an Option - so maybe it's not such a great idea after all?