Prevent rendering of unchanged default variables.

[MartijnCuppens]

Continuing on the idea @mdo dropped here: #27995 (comment). It would be nice if we can make some properties variable, but don't render them if the default values are kept.

I'm not sure if we can find a solution that wouldn't overcomplicate things too much, but just in case somebody has a brilliant idea, let us know!

[MartijnCuppens]

This was way easier than I tought, we can use just use null for this. For example see #28035