Fix tag overrides’ default_html preventing real tag usage

[thibaudcolas]

Description

This fixes a bug with our tag overrides’ support for default values. As implemented currently, we can no longer just let the tag render as normal – the default value would always take precedence, even if unset.

Even though this is a bug fix, I believe it might be sensible to only release this in a major version. Lots of projects might rely on the fact that tags always render None.

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• ~[ ] I have made corresponding changes to the documentation~ Not needed
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• ~[ ] I have added an appropriate CHANGELOG entry~ Will do this once merged

[cclauss]

This branch has conflicts that must be resolved.