input: Write a python function to find common characters between two strings.
  assert find_common_chars('Hello', 'World') == 'lo'
  assert find_common_chars('Python', 'JavaScript') == 't'
  assert find_common_chars('abcdef', 'fedcba') == 'abcdef'
  assert find_common_chars('abc', 'def') == ''
output: def find_common_chars(str1, str2):
      return ''.join(sorted(set(str1) & set(str2)))