Copy nested array into plain one when it is only empty

Question

How do I copy one nested array (urls2.url_breed) into an empty array url_alternative only if this one is empty like in this case? There are some cases when it is not empty and I am not interested to touch it,

{
  "breed": "Cavalier King Charles Spaniel",
  "url_alternative": [],
  "urls2": [
    {
      "_id": {
        "$oid": "627c04199569b8c2f566d1d3"
      },
      "breed": "Cavalier King Charles Spaniel",
      "url_breed": [
        "url1", "url2"
      ]
    }
  ]
}

Desired output:

{
  "breed": "Cavalier King Charles Spaniel",
  "url_alternative": [
        "url1" , "url2"
],
  "urls2": [
    {
      "_id": {
        "$oid": "627c04199569b8c2f566d1d3"
      },
      "breed": "Cavalier King Charles Spaniel",
      "url_breed": [
        "url1", "url2"
      ]
    }
  ]
}