The shipping address contains `null` values when there's no shipping address set on a cart

[dplaton]

Preconditions (*)

1. Magento 2.3.2

Steps to reproduce (*)

1. Use your favorite GraphQL client to create a guest cart using the createEmptyCart mutation
2. Request the cart details (just the shipping address should suffice):

  cart(cart_id: "YaMkSTdY7fOlShTKVJTNCLZHL2dtAWpT") {
    shipping_addresses {
      city
      company
      country {
        code
      }
      firstname
      lastname
      postcode
      region {
        code
      }
      street
      telephone
    }
  }
}

Expected result (*)

1. The shipping address in the response is an empty array

"data": {
    "cart": {
      "shipping_addresses": [ ]
    }
  }

Actual result (*)

1. The shipping address is an object with null values

  "data": {
    "cart": {
      "shipping_addresses": [
        {
          "city": null,
          "company": null,
          "country": {
            "code": null
          },
          "firstname": null,
          "lastname": null,
          "postcode": null,
          "region": {
            "code": null
          },
          "street": [
            ""
          ],
          "telephone": null
        }
      ]
    }
  }