input: Write a function to check whether a string contains only digits. If the string contains only digits, return True; otherwise, return False. If the string is empty, return True as well.
  assert is_digit_only("0123") == True
  assert is_digit_only("0123a") == False
  assert is_digit_only("") == True
  assert is_digit_only("987654321") == True
  assert is_digit_only("123.45") == False
output: def is_digit_only(s):
      return s.isdigit()