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.

从 System trait 的定义开始，检查由 feature 控制的 refresh_hotpatch 方法。检查启用 hotpatching 时手动实现的 System trait 会受到怎样的影响；当不需要 hotpatching 的实现无需定义此方法即可编译，并且相关检查或编译路径通过时，即视为完成。

由索引模型根据 Issue 内容生成。