Omit empty merge field choices array to pass API validation

[nmec]

Currently it is not possible to create new merge fields that are not options as the Mailchimp API does not accept null as an empty array, however it is possible to omit the field and pass validation.

Consider the struct:

gochimp3.MergeField{
  Tag: "My example field",
  Type: "text",
}

Will be encoded by json.Marshal as:

{
  "tag": "",
  "name": "My example field",
  "type": "text",
  "required": false,
  "default_value": "",
  "public": false,
  "display_order": 0,
  "options": {
    "default_Country": 0,
    "phone_format": "",
    "date_format": "",
    "choices": null,
    "size": 0
  },
  "help_text": ""
}

However Mailchimp will reject this request with an error, as choices is null rather than an empty array: []:

Invalid Resource : The resource submitted could not be validated. For field-specific details, see the 'errors' array. : [{options.choices Schema describes array, NULL found instead}]

There is an open issue on Go discussing the options for encoding as an empty array instead of null - golang/go#27589 - but in the meantime it is possible to omit the empty choices field and Mailchimp will create the merge field as expected.

[nmec]

Anything we need to do to get this merged @zeekay @davidtai ?

[zeekay]

Looks good! Merged in.