input: Write a python function to parse an HTML document and return the first <div> element using BeautifulSoup.
  assert get_first_div('<html><body><div>First div</div><div>Second div</div></body></html>') == '<div>First div</div>'
  assert get_first_div('<html><body><p>No div here</p></body></html>') is None
  assert get_first_div('<html><body><div class="test">First div</div><div>Second div</div></body></html>') == '<div class="test">First div</div>'
output: def get_first_div(html):
      from bs4 import BeautifulSoup
      
      soup = BeautifulSoup(html, 'html.parser')
      first_div = soup.find('div')
      
      return str(first_div) if first_div else None