[MC-122305] Cullface on the tops and bottoms of glass panes causes crashing.

The bug

When adding cullface to the tops of bottoms of glass panes, the game crashes. It is yet to be discovered if this affects any other blocks.

How to reproduce

Create a resourcepack that tries to add cullface to the tops or bottoms of glass panes
Open the game
Place a glass pane, and then the game crashes.

Crash report

17w47a: crash-2017-11-23_15.17.05-client.txt

Description: Tesselating block model

java.lang.IllegalArgumentException: Cannot get property null as it does not exist in bal{block=minecraft:glass_pane, properties=[east, north, south, west]}
	at bal$a.c(SourceFile:174)
	at auw.c(SourceFile:65)
	at bal$a.c(SourceFile:380)
	at bzz.c(SourceFile:94)
	at bzz.a(SourceFile:44)
	at bzx.a(SourceFile:62)
	at ccb.b(SourceFile:180)
	at cbx.a(SourceFile:100)
	at cbw.b(SourceFile:140)
	at bzj.a(SourceFile:925)
	at bzc.b(SourceFile:1158)
	at bzc.a(SourceFile:1093)
	at bzc.a(SourceFile:951)
	at bmg.b(SourceFile:810)
	at bmg.a(SourceFile:369)
	at net.minecraft.client.main.Main.main(SourceFile:134)