Fix unused variable warning for simple AssetV2 derives (bevyengine#9961)

# Objective

Fix bevyengine#9960 

## Solution

Make the `visit` parameter `_visit` if there are no dependencies.

New `cargo expand` output:
```rust
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")
    }
}
```

crates/bevy_asset/macros/src/lib.rs

@@ -66,9 +66,16 @@ fn derive_dependency_visitor_internal(
     let struct_name = &ast.ident;
     let (impl_generics, type_generics, where_clause) = &ast.generics.split_for_impl();
 
+    // prevent unused variable warning in case there are no dependencies
+    let visit = if field_visitors.is_empty() {
+        quote! { _visit }
+    } else {
+        quote! { visit }
+    };
+
     Ok(quote! {
         impl #impl_generics #bevy_asset_path::VisitAssetDependencies for #struct_name #type_generics #where_clause {
-            fn visit_dependencies(&self, visit: &mut impl FnMut(#bevy_asset_path::UntypedAssetId)) {
+            fn visit_dependencies(&self, #visit: &mut impl FnMut(#bevy_asset_path::UntypedAssetId)) {
                 #(#field_visitors)*
             }
         }