stylecontext: Don't do anything with empty widget path
stylecontext: Don't do anything with empty widget path

It's a corner case, but empty widget paths don't allow setting
state/regions/classes, so exit early

Fixes gtk/stylecontext test.

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