Improve pbr example (confusing comment)

[mdickopp]

Objective

After a recent fix for a panic in the pbr example (#16976), the code contains the following comment:

// This system relies on system parameters that are not available at start
// Ignore parameter failures so that it will run when possible
.add_systems(Update, environment_map_load_finish.never_param_warn())

However, this explanation is incorrect. EnvironmentMapLabel is available at start. The real issue is that it is no longer available once it has been removed by environment_map_load_finish.

Solution

• Remove confusing/incorrect comment and never_param_warn().
• Make Single<Entity, With<EnvironmentMapLabel>> optional in environment_map_load_finish, and check that the entity has not yet been despawned.

Since it is expected that an entity is no longer there once it has been despawned, it seems better to me to handle this case in environment_map_load_finish.

Testing

Ran cargo run --example pbr.