input: Write a function to determine if a 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("0123")==True
  assert is_digit_only("012a3")==False
  assert is_digit_only("")==True
  assert is_digit_only("12345")==True
  assert is_digit_only("123.45")==False
output: def is_digit_only(s):
      return s.isdigit()