Add missing return in default Relationship::on_insert impl (#17675)

# Objective

There was a bug in the default `Relationship::on_insert` implementation
that caused it to not properly handle entities targeting themselves in
relationships. The relationship component was properly removed, but it
would go on to add itself to its own target component.

## Solution

Added a missing `return` and a couple of tests
(`self_relationship_fails` failed on its second assert prior to this
PR).

## Testing

See above.

crates/bevy_ecs/src/relationship/mod.rs

@@ -83,6 +83,7 @@ pub trait Relationship: Component + Sized {
                 core::any::type_name::<Self>()
             );
             world.commands().entity(entity).remove::<Self>();
+            return;
         }
         if let Ok(mut target_entity_mut) = world.get_entity_mut(target_entity) {
             if let Some(mut relationship_target) =
@@ -276,4 +277,39 @@ mod tests {
         let c = world.spawn(Likes(a)).id();
         assert_eq!(world.entity(a).get::<LikedBy>().unwrap().0, &[b, c]);
     }
+
+    #[test]
+    fn self_relationship_fails() {
+        #[derive(Component)]
+        #[relationship(relationship_target = RelTarget)]
+        struct Rel(Entity);
+
+        #[derive(Component)]
+        #[relationship_target(relationship = Rel)]
+        struct RelTarget(Vec<Entity>);
+
+        let mut world = World::new();
+        let a = world.spawn_empty().id();
+        world.entity_mut(a).insert(Rel(a));
+        assert!(!world.entity(a).contains::<Rel>());
+        assert!(!world.entity(a).contains::<RelTarget>());
+    }
+
+    #[test]
+    fn relationship_with_missing_target_fails() {
+        #[derive(Component)]
+        #[relationship(relationship_target = RelTarget)]
+        struct Rel(Entity);
+
+        #[derive(Component)]
+        #[relationship_target(relationship = Rel)]
+        struct RelTarget(Vec<Entity>);
+
+        let mut world = World::new();
+        let a = world.spawn_empty().id();
+        world.despawn(a);
+        let b = world.spawn(Rel(a)).id();
+        assert!(!world.entity(b).contains::<Rel>());
+        assert!(!world.entity(b).contains::<RelTarget>());
+    }
 }