input: Write a function to extract URLs from HTML content using BeautifulSoup.
  assert extract_urls('<a href="http://example.com">Example</a>') == ['http://example.com']
  assert extract_urls('<div><a href="http://example1.com">Example1</a><a href="http://example2.com">Example2</a></div>') == ['http://example1.com', 'http://example2.com']
  assert extract_urls('<a>No URL here</a>') == []
  assert extract_urls('') == []
output: def extract_urls(html_content):
      from bs4 import BeautifulSoup
  
      soup = BeautifulSoup(html_content, 'html.parser')
      return [a.get('href') for a in soup.find_all('a', href=True)]