input: Write a function to remove all HTML tags from a given string. The function should return the cleaned string with all HTML tags removed. If the input string is empty, the function should 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='example.com'>Link</a>")=='Link'
  assert clean_html_tags("<h1>Title</h1>")=='Title'
  assert clean_html_tags("No tags here")=='No tags here'
  assert clean_html_tags("")==''
output: def clean_html_tags(s):
      import re
      return re.sub(r'<[^>]+>', '', s) if s else ''