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

Documentation
2

Something like this

upsert {
  query {
    q(func: eq(email, "user@company1.io")) {
      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/