zone: make Owner fields optional

[thomasqueirozb]

According to the docs id and name are not required and email isn't even mentioned. My request had id and email as null, which broke deserialization

      "owner": { "id": null, "type": "user", "email": null },

~The correct thing to do would be to make these fields Option<String> but I didn't do so in order not to break the api.~ Had to convert the fields to Option<String>, which breaks the public API, but I doubt anyone is using the Owner struct tbh.

This is also how the go implementation does it too:

https://github.com/cloudflare/cloudflare-go/blob/325729d2e358cd3f300c130cfa828e5704e725d8/zone.go#L24-L29

If any of the fields are null, the go json library will just make that field an empty string.