I am given the below json and it needs to be concatenated based on condition if it not null

Can someone help me out here?

expected output

{
  "id": "4583",
  "price": "149.99 EUR",
  "sale_price": ""
}

In above json price was concatenated with EUR but sale_price is as it is because it is null.

I am using below jolt, it us updating sale_price as well.

{
    "operation": "modify-overwrite-beta",
    "spec": {
      "price": "=concat(@(1,price),' USD')",
      "sale_price": ["=isNull", "=concat(@(1,sale_price),' USD')"]
    }
  }