Handle empty idlist in MigrateSourceMultiItems

Problem/Motivation

If you dont have empty list of xmls for import by using MigrateSourceMultiItems

  /**
   * Implementation of MigrateSource::performRewind().
   *
   * @return void
   */
  public function performRewind() {
    // If there isn't a specific ID list passed in, get it from the list class.
    if ($this->idList) {
      $this->idsToProcess = $this->idList;
    }
    else {
      $this->idsToProcess = $this->itemsClass->getIdList();
    }
    $this->idIterator = ($this->idsToProcess instanceof Iterator) ?
      $this->idsToProcess : new ArrayIterator($this->idsToProcess);
    $this->idIterator->rewind();
  }

Proposed resolution

patch