Make zone sync struct optional

[Rulox]

Proposed changes

NGINX Plus does not return StreamZoneSync metrics if there is no sync zone configured. Because of this, we need to make these a pointer to the struct so we can check if the metrics are present or not (instead of returning an empty struct with zeroes)

This will require the user to be careful when using the Stats.StreamZoneSync field, as it can be nil.

[pleshakov]

@Rulox looks good!

small comment: do we need to change the tests https://github.com/nginxinc/nginx-plus-go-client/blob/master/tests/client_test.go#L770 and https://github.com/nginxinc/nginx-plus-go-client/blob/master/tests/client_test.go#L806 so that we check if StreamZoneSync is not nil and fail if it is?

[Rulox]

Yes we can add those. That will never happen, the test will fail anyways because the struct is nil. Again this will never happens unless we change the nginx configuration. I'll add the checks for a better error message tho 👍