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. If the string is empty, return False.
  
  assert is_digit_only("0123")==True
  assert is_digit_only("abc123")==False
  assert is_digit_only("")==False
  assert is_digit_only("456")==True
  assert is_digit_only("789a")==False
output: def is_digit_only(s):
      return s.isdigit() if s else False