__browser_default__ does not let you easily return None

[Andy McKay]

Browser default lets you change the default page from index_html to something more flexible. However if the page you want to return does not exist, its hard to just ask the browser to return __call__ on the folder because: returning [None,] or ['',] makes an infinite loop and returning ['__call__,] is a security error and [getattr(content, '__call__'),] is not allowed in a Script (Python) (although it would be a Python Product.

The following patch just adds one more case that might be useful if you return [], then it goes and calls the default.

Im adding it here rather than checking it in, because this is not a hugely desirable or great thing and I found a workaround for the CMF. However I thought this might be useful to leave a paper trail.

[Colin Watson (cjwatson)]

The zope2 project on Launchpad has been archived at the request of the Zope developers (see https://answers.launchpad.net/launchpad/+question/683589 and https://answers.launchpad.net/launchpad/+question/685285). If this bug is still relevant, please refile it at https://github.com/zopefoundation/zope2.