Release type: patch

Set default content status to a blank string rather than `None`. Fixes
[#2558](https://github.com/getpelican/pelican/issues/2558). Fixes issues
encountered by comment plugins among others
([1](https://github.com/bstpierre/pelican-comments/pull/4),
[2](https://github.com/Scheirle/pelican_comment_system/issues/8)).