input: Write a function to evaluate a postfix expression given as a string. The function should return the evaluated value of the expression. If the input expression is empty, the function should return None.
  
  def evaluate(expression): 
      if not expression:
          return None
      
      expression = expression.split() 
      stack = []  
    
      for element in expression: 
          if element.isdigit(): 
              stack.append(element) 
          else:  
              val1 = stack.pop() 
              val2 = stack.pop() 
              stack.append(str(eval(val2 + element + val1))) 
    
      return (int(stack[0])) 
  
  assert evaluate("2 3 + 4 5 * *")==46
  assert evaluate("3 4 + 2 * 7 /")==2
  assert evaluate("5 3 4 2 - * +")==17
  assert evaluate("10 6 9 3 + -11 * / * 17 + 5 +")==22
  assert evaluate("")==None
output: def evaluate(expression): 
      if not expression:
          return None
      
      expression = expression.split() 
      stack = []  
    
      for element in expression: 
          if element.isdigit() or (element[0] == '-' and element[1:].isdigit()): 
              stack.append(int(element)) 
          else:  
              val1 = stack.pop() 
              val2 = stack.pop() 
              if element == '+':
                  stack.append(val2 + val1)
              elif element == '-':
                  stack.append(val2 - val1)
              elif element == '*':
                  stack.append(val2 * val1)
              elif element == '/':
                  stack.append(int(val2 / val1))
    
      return stack[0]