input: Write a function to clean a string of HTML tags and return the cleaned string. If the input string is empty, return an empty string.
  assert clean_html_tags("<p>This is a <b>test</b> string</p>")=="This is a test string"
  assert clean_html_tags("<a href='url'>Link</a>")=="Link"
  assert clean_html_tags("No tags here")=="No tags here"
  assert clean_html_tags("")==""
output: def clean_html_tags(s):
      import re
      if not s:
          return ""
      return re.sub(r'<[^>]+>', '', s)