input: Write a function to check whether a string contains only digits. The function should return True if the string contains only digits, otherwise return False. The function should handle an empty string and return True if the string is empty.
  assert is_digit_only("0123")==True
  assert is_digit_only("123abc")==False
  assert is_digit_only("9876543210")==True
  assert is_digit_only("123.45")==False
  assert is_digit_only("")==True
output: def is_digit_only(s):
      return s.isdigit() or s == ""