core: impl field::Value for Option<T>

[DCjanus]

Motivation

Build a span with some field might be None is bother now.

Before:

let span = info!("example", data = Empty );
if let Some(data) = foo() {
    span.record("data", &data);
}

After:

let span = info!("example", data = foo() );

[hawkw]

thanks, looks good to me.

it would be nice to have some tests for this, if you have the time?

[liamdawson]

This would close #824, right?

[hawkw]

merged in #1585.