Allow deserialize_any to call visit_none on empty cells

[msrd0]

I'm trying to deserialize Option<chrono::NaiveTime> from inside a custom deserialize implementation that calls deserialize_map and then next_key/next_value with all type info available. Nevertheless, it ends up calling deserialize_any instead of deserialize_option, and then fails to parse on empty cells instead of returning None as I'd expect. This PR allows deserialize_any to treat empty fields as None.