feature-gated `System::refresh_hotpatch` with no default impl is inconvenient for crate authors

When the user uses the `hotpatching` feature, the `System` trait has one additional method, [`refresh_hotpatch`](https://dev-docs.bevy.org/bevy/ecs/prelude/trait.System.html#tymethod.refresh_hotpatch).

However, this method has no default impl. This means that other 3rd party crates the user relies on need to offer that feature too if these manually implement `System`, even if hotpatching makes no sense for them. If they don't do this, the user's project wont compile.

A solution could be to add an empty body as the default impl.

Start at the System trait definition and inspect the feature-gated refresh_hotpatch method. Check how manually implemented System traits are affected when hotpatching is enabled; done means implementations that do not need hotpatching can compile without defining this method, with the relevant checks or compilation paths passing.

Scritto dal modello di indicizzazione a partire dal testo della issue.