input: Write a function to extract titles and descriptions from given HTML content using scrapy selectors.
  assert extract_data('<div><item><item-title>Title1</item-title><item-description>Description1</item-description></item></div>') == [{'title': 'Title1', 'description': 'Description1'}]
  assert extract_data('<div><item><item-title>Title2</item-title><item-description>Description2</item-description></item><item><item-title>Title3</item-title><item-description>Description3</item-description></item></div>') == [{'title': 'Title2', 'description': 'Description2'}, {'title': 'Title3', 'description': 'Description3'}]
  assert extract_data('<div></div>') == []
output: def extract_data(html_content):
      from scrapy.selector import Selector
      selector = Selector(text=html_content)
      items = selector.css('item')
      result = []
      for item in items:
          title = item.css('item-title::text').get()
          description = item.css('item-description::text').get()
          if title and description:
              result.append({'title': title, 'description': description})
      return result