Querying array of references in Sanity schema and resolving null values

*[_type == "post" && slug.current == 'hello-world'] | order(_updatedAt desc) {
  title,
  'test': countries[].country->{title, slug}
}

{
  name: "post",
  title: "Posts",
  type: "document",
  fields: [
    {
      name: 'title',
      title: 'Title',
      type: 'string',
    },
    {
      type: 'array',
      name: 'countries',
      title: 'Countries',
      of: [{
        type: 'object',
        fields: [
          {
            name: 'country',
            title: 'Country',
            type: 'reference',
            to: [{ type: 'countries' }],
          },
        ],
      }],
    },
  ],
}

{
  name: 'countries',
  title: 'Countries',
  type: 'array',
  of: [{ type: 'reference', to: { type: 'country' } }],
}, 

*[_type == "post" && slug.current == 'hello-world'] | order(_updatedAt desc) {
  title,
  'test': countries[]->{title, slug}
}