[MC-116927] Server crashes with "null" as advancement title or description value

The bug

Setting the title or description of an advancement to null crashes the server.

title-null-crash-2017-05-09_15.09.31-server.txt

Description: Exception in server tick loop

java.lang.NullPointerException: Exception in server tick loop
	at i.<init>(SourceFile:51)
	at i$a.a(SourceFile:170)
	at j.a(SourceFile:65)
	at ng.a(SourceFile:74)
	at ng.<init>(SourceFile:66)
	at ob.b(SourceFile:162)
	at cgl.a(SourceFile:98)
	at cgl.j(SourceFile:130)
	at net.minecraft.server.MinecraftServer.run(SourceFile:436)
	at java.lang.Thread.run(Unknown Source)

description-null-crash-2017-05-09_15.20.53-server.txt

Description: Exception in server tick loop

java.lang.NullPointerException: Exception in server tick loop
	at gw.a(SourceFile:18)
	at i.<init>(SourceFile:54)
	at i$a.a(SourceFile:170)
	at j.a(SourceFile:65)
	at ng.a(SourceFile:74)
	at ng.<init>(SourceFile:66)
	at ob.b(SourceFile:162)
	at cgl.a(SourceFile:98)
	at cgl.j(SourceFile:130)
	at net.minecraft.server.MinecraftServer.run(SourceFile:436)
	at java.lang.Thread.run(Unknown Source)

Note: It is likely that ~~MC-87587~~ and ~~MC-92867~~ can cause a crash as well.

How to reproduce

Download the attached advancement title_description_null.json and place it in the following directory of your world folder
```
data\advancements\custom
```
Load the world