input: Write a function to remove all HTML tags from a given string. The function should return a clean string without any HTML tags. If the input string is empty, the function should return an empty string.
  def clean_html_tags(str):
      clean_str = re.sub("<.*?>", "", str) if str else ""
      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("Hello World") == "Hello World"
  assert clean_html_tags("") == ""
  assert clean_html_tags("<a>Link</a>") == "Link"
  assert clean_html_tags("<h1>Title</h1><p>Paragraph1</p><p>Paragraph2</p>") == "TitleParagraph1Paragraph2"
output: def clean_html_tags(str):
      clean_str = re.sub("<.*?>", "", str) if str else ""
      return clean_str