input: Write a function to check if a given string contains only digits. If the string contains only digits, return True; otherwise, return False. If the given string is empty, return True.
  
  assert is_digit_only("0123")==True
  assert is_digit_only("0abc")==False
  assert is_digit_only("123456")==True
  assert is_digit_only("")==True
output: def is_digit_only(s):
      return s.isdigit() or s == ""