TypeError in getHtml

Hi!

Nice library.

I have only one small problem. When i try print tag which contain attribute with None value, print fail.

Here is simple code to reproduce that:

import AdvancedHTMLParser

parser = AdvancedHTMLParser.AdvancedHTMLParser()
parser.parseStr('<div itemscope itemtype="http://schema.org/Order">')
parser.getHTML()

Returnung:

Traceback (most recent call last):
  File "/home/jiri/userconfig/mail/error.py", line 5, in <module>
    parser.getHTML()
  File "/tmp/p/venv/lib/python3.11/site-packages/AdvancedHTMLParser/Parser.py", line 887, in getHTML
    return doctypeStr + rootNode.outerHTML
           ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
TypeError: can only concatenate str (not "NoneType") to str

Observed on python 3.11.5 (Arch linux) with up-to-date package from pip.