input: Write a function to determine if a given string contains only digits. If the string contains only digits, return True; otherwise, return False. If the string is empty, return True.
  assert is_digit_only("0123")==True
  assert is_digit_only("abc123")==False
  assert is_digit_only("")==True
  assert is_digit_only("123.45")==False
  assert is_digit_only("9876543210")==True
output: def is_digit_only(s):
      return s.isdigit()