code: private function buildCursorFromFirstItem(CursorStrategyInterface $cursorStrategy, Result $result)
      {
          if (0 === $result->get('Count')) {
              return null;
          }
  
          $items     = $result->get('Items');
          $firstItem = $this->marshaler->unmarshalItem(reset($items));
  
          return $cursorStrategy->buildCursorFromKey($firstItem);
      }
docs: @param CursorStrategyInterface $cursorStrategy
  @param Result                  $result
  
  @return null|string