input: Write a python function to extract the title from an HTML document using BeautifulSoup.
  assert extract_title('<html><head><title>Test Page</title></head><body></body></html>') == 'Test Page'
  assert extract_title('<html><head><title>Another Test</title></head><body></body></html>') == 'Another Test'
  assert extract_title('<html><head><title></title></head><body></body></html>') == ''
output: def extract_title(html_document):
      from bs4 import BeautifulSoup
      soup = BeautifulSoup(html_document, 'html.parser')
      title_tag = soup.find('title')
      return title_tag.string if title_tag else ''