Systems should be skipped if their resources cannot be fetched #15265
Description
> The solution to this I would prefer is to lean into the ECS paradigm more & treat more things as if they were querying the world. If a Res<Foo> doesn't exist the system just doesn't run. Similar to how iterating over a Query<Bar> that matches nothing loops over nothing. If users want a more explicit panicky behavior they can use Option<Res<Foo>> & unwrap.
I do think we should do this, and if we can get this behavior in place I'd prefer to encourage the use of the Single system param from #15264. This would go a long way to addressing the problem and ensuring non-panicking behavior by default without an ergonomics hit.
Originally posted by @iiYese and @alice-i-cecile in #14275 (comment)