serde serialization of crs and edges should be skipped when None

[JosiahParry]

Per https://github.com/geoarrow/geoarrow-r/issues/34#issuecomment-1925872634 crs and edges shouldn't return null when not set. Rather, they should be omitted. This can be configured with serde #[serde(skip_serializing_if = "Option::is_none")] on the Option field

https://github.com/geoarrow/geoarrow-rs/blob/eb94df3d998801be3c55128b4535fc9f58652b66/src/array/metadata.rs#L28-L32

[kylebarron]

good to know about that serde option but we should be checking if all fields are null and not even writing the metadata key in that case

[kylebarron]

This is solved now. Helped by https://github.com/geoarrow/geoarrow-rs/pull/719 but solved separately before that I think.