Face `:background` property should be set to `unspecified` instead of `nil`

I'm guessing that setting properties to `nil` was maybe once accepted, but it's triggering warnings now I'm getting an error from here, https://github.com/minad/org-modern/blob/1723689710715da9134e62ae7e6d41891031813c/org-modern.el#L688C54-L688C68 > Warning: setting attribute ‘:background’ of face ‘default’: nil value is invalid, use ‘unspecified’ instead. But from searching there's a few instances of `:background nil` and `:foreground nil` that probably need fixing too. I have no clue if this will break old setups with old Emacs or org packages.