[MC-116663] Empty advancement file crashes server when loading world

The bug

Putting an empty .json file in the data\advancements folder of a world causes the server to crash when the world is loaded.

crash-2017-04-26_18.07.27-server.txt

Description: Exception in server tick loop

java.lang.NullPointerException: Exception in server tick loop
	at j.a(SourceFile:64)
	at mx.a(SourceFile:74)
	at mx.<init>(SourceFile:66)
	at ns.b(SourceFile:162)
	at cgb.a(SourceFile:98)
	at cgb.j(SourceFile:130)
	at net.minecraft.server.MinecraftServer.run(SourceFile:436)
	at java.lang.Thread.run(Unknown Source)