Change trait bounds of `NoneAsEmptyString`?

[jonasbb]

The serialize implementation relies on AsRef<str>. Maybe it is better to use the Display bound instead. This might make it more useable with non-String types, e.g., Option<i32>.

Here is a reference where it is used to parse get query parameter, which can be the empty string when not set:
tokio-rs/axum#443