input: Write a function to identify if a string has matching parentheses. If the given string is empty, then return True.
  assert matching_paren("()")==True
  assert matching_paren("(())")==True
  assert matching_paren("(()")==False
  assert matching_paren(")(")==False
  assert matching_paren("(())()")==True
  assert matching_paren("")==True
output: def matching_paren(s):
      stack = []
      for char in s:
          if char == '(':
              stack.append(char)
          elif char == ')':
              if not stack:
                  return False
              stack.pop()
      return len(stack) == 0