actor: Clear MetaGroups when empty
actor: Clear MetaGroups when empty

When removing the last Action, Constraint, or Effect, we should also be
clearing the corresponding MetaGroup: code inside ClutterActor relies on
NULL checks, and changing them all to check for NULL && n_items == 0
would not be fun.

GNOME/clutter,jigpu/clutter,djdeath/clutter-android,djdeath/clutter,GNOME/clutter,djdeath/clutter,kerrickstaley/clutter-vala,djdeath/clutter-android,djdeath/clutter-android,jigpu/clutter,heysion/clutter-clone,heysion/clutter-clone,heysion/clutter-clone,kerrickstaley/clutter-vala,ebassi/clutter,collects/clutter,ebassi/clutter,heysion/clutter-clone,GNOME/clutter,kerrickstaley/clutter-vala,ebassi/clutter,Distrotech/clutter,GNOME/clutter,ebassi/clutter,kerrickstaley/clutter-vala,collects/clutter,jigpu/clutter,djdeath/clutter-android,Distrotech/clutter,jigpu/clutter,djdeath/clutter-android,jigpu/clutter,djdeath/clutter,collects/clutter,djdeath/clutter,Distrotech/clutter,ebassi/clutter,Distrotech/clutter,collects/clutter,Distrotech/clutter,GNOME/clutter,djdeath/clutter,Distrotech/clutter,heysion/clutter-clone,djdeath/clutter,jigpu/clutter,collects/clutter