input: Write a function to check if one string is a substring of another.
  assert is_substring('hello world', 'world') == True
  assert is_substring('hello world', 'worlds') == False
  assert is_substring('', 'world') == False
  assert is_substring('hello world', '') == False
output: def is_substring(s1, s2):
      if s1 == '' or s2 == '':
          return False
      return s2 in s1