input: Write a function to check whether a given string contains only digits. Return True if the string contains only digits, otherwise return False. If the string is empty, return True.
  assert is_digit_only("123456")==True
  assert is_digit_only("123a56")==False
  assert is_digit_only("")==True
  assert is_digit_only("0")==True
  assert is_digit_only("abc123")==False
output: def is_digit_only(s):
      return s.isdigit()