Fix unused variable warning for simple AssetV2 derives

[ItsDoot]

Objective

Fix #9960

Solution

Make the visit parameter _visit if there are no dependencies.

New cargo expand output:

pub struct Rarity {
    pub name: SharedStr,
    pub color: Color,
}
impl bevy::asset::Asset for Rarity {}
impl bevy::asset::VisitAssetDependencies for Rarity {
    fn visit_dependencies(
        &self,
        _visit: &mut impl FnMut(bevy::asset::UntypedAssetId), // <-- fixed
    ) {}
}
impl bevy::reflect::TypePath for Rarity {
    fn type_path() -> &'static str {
        "myasset::item::Rarity"
    }
    fn short_type_path() -> &'static str {
        "Rarity"
    }
    fn type_ident() -> Option<&'static str> {
        ::core::option::Option::Some("Rarity")
    }
    fn crate_name() -> Option<&'static str> {
        ::core::option::Option::Some(
            "myasset::item".split(':').next().unwrap(),
        )
    }
    fn module_path() -> Option<&'static str> {
        ::core::option::Option::Some("myasset::item")
    }
}