avoid panic with parented scenes on deleted entities

[robtfm]

Objective

after calling SceneSpawner::spawn_as_child, the scene spawner system will always try to attach the scene instance to the parent once it is loaded, even if the parent has been deleted, causing a panic.

Solution

check if the parent is still alive, and don't spawn the scene instance if not.

[mockersf]

This seems not the best fix for me as it means we still spawn the scene and one frame after despawn it.

To avoid that, a check could be added in this system: https://github.com/bevyengine/bevy/blob/main/crates/bevy_scene/src/scene_spawner.rs#L317