[MC-116926] Advancement file containing only "null" crashes server

The bug

Providing an advancement which only contains null crashes the server when loading the world.

crash-2017-05-02_21.32.11-server.txt

Description: Exception in server tick loop

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