Fix adding outline when SceneSpawner is not in the world

Bevy removes it from the world when a scene is spawned via
`SceneSpawner`:
https://github.com/bevyengine/bevy/blob/f667c282dad2c1419afb5836ded22a3ec263970e/crates/bevy_scene/src/scene_spawner.rs#L560

If `AsyncSceneInheritOutline` is inserted during the scene
spawning (via a required component or via trigger), the
system won't run since it requires `SceneSpawner`.

Tests use `DynamicSceneRoot` instead of using `SceneSpawner`
directly and it works since it uses a bit different mechanism.

I noticed this in my game and tested the fix.

Author: Shatur

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "bevy_mod_outline"
 version = "0.12.0"
-edition = "2021"
+edition = "2024"
 license = "MIT OR Apache-2.0"
 description = "A mesh outlining plugin for Bevy."
 readme = "README.md"

src/scene.rs

@@ -41,13 +41,15 @@ fn add_outline(
     mut commands: Commands,
     mut query: Query<(&mut AsyncSceneInheritOutline, Option<&SceneInstance>)>,
     systems: Res<AsyncSceneInheritOutlineSystems>,
-    scene_spawner: Res<SceneSpawner>,
+    scene_spawner: Option<Res<SceneSpawner>>, // Could be temporarily removed from the world when a scene is spawning
 ) {
     let Ok((mut scene_outline, scene_instance)) = query.get_mut(*entity_input) else {
         return;
     };
     let mut ready = false;
-    if let Some(scene_instance) = scene_instance {
+    if let Some(scene_instance) = scene_instance
+        && let Some(scene_spawner) = scene_spawner
+    {
         let iid = **scene_instance;
         if scene_spawner.instance_is_ready(iid) {
             for child in scene_spawner.iter_instance_entities(iid) {