[MC-88764] JSON signs with scoreboard objectives in them regenerate the chunks when the world is reloaded

Whenever a JSON sign with a scoreboard objective is loaded into the world, the chunk that it is in is completely regenerated back to the state it was determined by the world seed.

Steps to reproduce:

1. load a pre-1.9 map with JSON signs in it that have scoreboard objectives
2. Notice how the chunks are completely reset; the map should look garbled now

Command used

/blockdata ~ ~ ~1 {Text1:"[\"\",{\"text\":\"hi\"}]",Text2:"[\"\",{\"score\":{\"name\":\"@p\",\"objective\":\"hi\"}}]"}

Exception in game output

[02:16:15] [Server thread/ERROR]: Couldn't load chunk
java.lang.NullPointerException
	at aph$1.h(SourceFile:101) ~[15w37a.jar:?]
	at ex.a(SourceFile:38) ~[15w37a.jar:?]
	at ev.a(SourceFile:31) ~[15w37a.jar:?]
	at ev.a(SourceFile:57) ~[15w37a.jar:?]
	at aph.a(SourceFile:110) ~[15w37a.jar:?]
	at aoq.c(SourceFile:105) ~[15w37a.jar:?]
	at arn.a(SourceFile:322) ~[15w37a.jar:?]
	at arn.a(SourceFile:83) ~[15w37a.jar:?]
	at arn.a(SourceFile:70) ~[15w37a.jar:?]
	at lj.f(SourceFile:124) [15w37a.jar:?]
	at lj.c(SourceFile:76) [15w37a.jar:?]
	at lj.d(SourceFile:91) [15w37a.jar:?]
	at net.minecraft.server.MinecraftServer.l(SourceFile:324) [15w37a.jar:?]
	at bvm.a(SourceFile:114) [15w37a.jar:?]
	at bvm.j(SourceFile:130) [15w37a.jar:?]
	at net.minecraft.server.MinecraftServer.run(SourceFile:420) [15w37a.jar:?]
	at java.lang.Thread.run(Thread.java:745) [?:1.8.0_60]