levelbar: don't use the current value style class for empty blocks
levelbar: don't use the current value style class for empty blocks

We want to be able to style the empty blocks independently of all the
offset styles, so remove the current style class when painting an empty
block.

https://bugzilla.gnome.org/show_bug.cgi?id=707695

alexlarsson/gtk,jigpu/gtk,grubersjoe/adwaita,Lyude/gtk-,Lyude/gtk-,grubersjoe/adwaita,Lyude/gtk-,Lyude/gtk-,ahodesuka/gtk,davidgumberg/gtk,chergert/gtk,jigpu/gtk,grubersjoe/adwaita,grubersjoe/adwaita,ahodesuka/gtk,grubersjoe/adwaita,davidgumberg/gtk,jadahl/gtk,chergert/gtk,Lyude/gtk-,alexlarsson/gtk,jigpu/gtk,davidgumberg/gtk,jadahl/gtk,jadahl/gtk,Adamovskiy/gtk,alexlarsson/gtk,jadahl/gtk,ahodesuka/gtk,jigpu/gtk,Lyude/gtk-,chergert/gtk,alexlarsson/gtk,grubersjoe/adwaita,jigpu/gtk,davidgumberg/gtk,jigpu/gtk,chergert/gtk,ahodesuka/gtk,davidgumberg/gtk,Adamovskiy/gtk,Adamovskiy/gtk,chergert/gtk,chergert/gtk,ahodesuka/gtk,alexlarsson/gtk,ahodesuka/gtk,alexlarsson/gtk,davidgumberg/gtk,Lyude/gtk-,Adamovskiy/gtk,chergert/gtk,jadahl/gtk,grubersjoe/adwaita,Adamovskiy/gtk,Adamovskiy/gtk,Adamovskiy/gtk,chergert/gtk,jadahl/gtk,jadahl/gtk,jadahl/gtk,alexlarsson/gtk,jigpu/gtk,jigpu/gtk,davidgumberg/gtk,Lyude/gtk-,davidgumberg/gtk,ahodesuka/gtk,alexlarsson/gtk,Adamovskiy/gtk,ahodesuka/gtk,grubersjoe/adwaita