Empty new_parents list is created but should just be omitted

[TheJJ]

Somehow, a new_parents vector is created in a Transaction even there are no new parents for an object.

Find the bug and prevent that new_parents is added to the changed-map when there are no new parents.

### p tx
$19 = {
  error = {
    _M_exception_object = 0x0
  },
  valid = true,
  at = 0,
  states = std::vector of length 1, capacity 1 = {
    std::tuple containing = {
      [1] = std::shared_ptr (count 5, weak 1) 0x645ea0,
      [2] = std::shared_ptr (count 1, weak 0) 0x644960,
      [3] = {
        changes = std::unordered_map with 1 element = {
          ["test.First"] = {
            new_parents = std::vector of length 0, capacity 0
          }
        }
      }
    }
  }
}