How to set value only if the value is not exist?

upsert {
  query {
    q(func: eq(email, "[email protected]")) {
      v as uid
    }
  }

  mutation {
    set {
      uid(v) <age> "28" .
    }
  }

in this example, the age will be updated no matter it already has a old value or not. how to value only if the value is not exist? such as uid(v) <age> val(age) || 28

Something like this

upsert {
  query {
    q(func: eq(email, "[email protected]")) {
      v as uid
    }
    var(func: uid(v)) @filter(eq(age, "28")) {
      v2 as uid
    }
  }

  mutation @if(eq(len(v2), 0)){
    set {
      uid(v) <age> "28" .
    }
  }

See https://dgraph.io/docs/mutations/conditional-upsert/