Allow optional settings

[julianrubisch]

I wonder if there's a case to be made for settings to not have a default. Right now maglev marks these as invalid, but consider this scenario:

I have a content section with an image, which I'd like to make optional, so the user can change whether or not to add an image. If the image is omitted, the respective HTML in the template is just not rendered, which works fine.

However, I would like to leave out the default image in the yml so the section defaults to empty. It's marked as invalid in the admin interface, but works fine. I wonder if the validation rules could/should be loosened?

The canonical solution would probably be to add two separate sections (with and without image), but I feel like this would be an overdo. WDYT?