units: Improve coverage of clutter_units_from_string()
units: Improve coverage of clutter_units_from_string()

Add a unit for an empty string as well as units for the missing unit
types like cm, mm and px.

dlespiau/clutter,spatulasnout/clutter,dlespiau/clutter,djdeath/clutter-multithreaded,kerrickstaley/clutter-vala,Distrotech/clutter,ebassi/clutter,heysion/clutter-clone,collects/clutter,heysion/clutter-clone,ebassi/clutter,kerrickstaley/clutter-vala,nobled/clutter,Distrotech/clutter,GNOME/clutter,rkudiyarov/clutter_osx,djdeath/clutter,jigpu/clutter,spatulasnout/clutter,Distrotech/clutter,jigpu/clutter,djdeath/clutter,GNOME/clutter,kerrickstaley/clutter-vala,Distrotech/clutter,jigpu/clutter,heysion/clutter-clone,djdeath/clutter-android,spatulasnout/clutter,jigpu/clutter,jigpu/clutter,ebassi/clutter,heysion/clutter-clone,spatulasnout/clutter,rkudiyarov/clutter_osx,jigpu/clutter,djdeath/clutter-android,djdeath/clutter-android,dlespiau/clutter,djdeath/clutter-android,ebassi/clutter,djdeath/clutter-multithreaded,collects/clutter,collects/clutter,GNOME/clutter,heysion/clutter-clone,ebassi/clutter,djdeath/clutter,nobled/clutter,rkudiyarov/clutter_osx,djdeath/clutter-multithreaded,djdeath/clutter-android,djdeath/clutter,GNOME/clutter,kerrickstaley/clutter-vala,nobled/clutter,Distrotech/clutter,spatulasnout/clutter,collects/clutter,djdeath/clutter,rkudiyarov/clutter_osx,Distrotech/clutter,djdeath/clutter-multithreaded,djdeath/clutter,collects/clutter,GNOME/clutter