为什么这个二叉树预序遍历返回none

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def preorderTraversal(root):
    if root is None:
        return None
    
    result = []
    result.append(root.val)
    result += preorderTraversal(root.left)
    result += preorderTraversal(root.right)
    
    return result