more than just `&self` in non-standard implementations.
changelog: Fix [`len_without_is_empty`] false positive when len has a
non-standard method signature
Fixes#9520
Check the return type of `len`. Only integral types, or an `Option` or `Result` wrapping one.
Ensure the return type of `is_empty` matches. e.g. `Option<usize>` -> `Option<bool>`