input: Write a function to clean a string of HTML tags. Remove all HTML tags from the input string and return the cleaned string. If the input string is empty, return an empty string.
  
  def clean_html_tags(str):
      if not str:
          return ""
      clean_str = re.sub("<.*?>", "", str)
      return clean_str
  
  assert clean_html_tags("<p>This is a <b>test</b> string</p>") == "This is a test string"
  assert clean_html_tags("No tags here") == "No tags here"
  assert clean_html_tags("") == ""
  assert clean_html_tags("<a>Link</a>") == "Link"
  assert clean_html_tags("<p>Start</p><div>Middle</div><span>End</span>") == "StartMiddleEnd"
output: def clean_html_tags(str):
      if not str:
          return ""
      clean_str = re.sub("<.*?>", "", str)
      return clean_str