input: Write a function to check whether a string contains only digits. If the string contains only digits return True else return False. If the string is an empty string then return True.
  assert is_digit_only("0123")==True
  assert is_digit_only("012a3")==False
  assert is_digit_only("12345")==True
  assert is_digit_only("")==True
  assert is_digit_only("123.45")==False
output: def is_digit_only(s):
      return s.isdigit()