input: Write a python function to get the HTML content of a given URL and return it as a string.
  assert get_html("https://www.example.com") is not None
  assert isinstance(get_html("https://www.example.com"), str)
  assert get_html("invalid_url") is None
output: def get_html(url):
      import requests
      try:
          response = requests.get(url)
          response.raise_for_status()
          return response.text
      except (requests.RequestException, ValueError):
          return None