input: Write a function to clean a string of HTML tags. The function should remove all HTML tags from the given string 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("<h1>Hello, World!</h1>")=="Hello, World!"
  assert clean_html_tags("<div><p>Content</p></div>")=="Content"
  assert clean_html_tags("")==""
  assert clean_html_tags("<span>Only Span</span>")=="Only Span"
output: def clean_html_tags(s):
      import re
      if not s:
          return ""
      return re.sub(r'<[^>]+>', '', s)