[MC-197225] Crash: java.util.concurrent.CompletionException: u: Exception generating new chunk

The game crashes when a biome doesn't include the spawners arrays.

Crash

"spawners": {}

No crash

"spawners": {
    "monster": [],
    "creature": [],
    "ambient": [],
    "water_creature": [],
    "water_ambient": [],
    "misc": []
},

Usually when certain fields are non optional, there is an error in the log and the biome doesn't load. However this is not the case here, and the game crashes.

I have also experienced this freezing the game as seen in the screenshots

How to reproduce

Download the empty_spawners.zip data pack and install it in a world
Teleport to the blip:test dimension
```
execute in blip:test run tp @s ~ ~ ~ 
```
The game crashes

1.16.2-pre2: crash-2020-08-06_15.24.44-server.txt

Time: 8/6/20 3:24 PM
Description: Exception in server tick loop

java.util.concurrent.CompletionException: u: Exception generating new chunk
	at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:273)
	at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:280)
	at java.util.concurrent.CompletableFuture.uniCompose(CompletableFuture.java:961)
	at java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:926)
	at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:442)
	at zu.b(SourceFile:58)
	at zu$$Lambda$4225/119868154.run(Unknown Source)
	at aob.e(SourceFile:94)
	at aob.a(SourceFile:137)
	at aob.run(SourceFile:105)
	at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1402)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1689)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)
Caused by: u: Exception generating new chunk
	at zs.a(SourceFile:579)
	at zs$$Lambda$4261/699495355.apply(Unknown Source)
	at com.mojang.datafixers.util.Either$Left.map(Either.java:38)
	at zs.a(SourceFile:562)
	at zs$$Lambda$4258/533721112.apply(Unknown Source)
	at java.util.concurrent.CompletableFuture.uniCompose(CompletableFuture.java:952)
	... 12 more
Caused by: java.lang.NullPointerException
	at bsd.a(SourceFile:460)
	at chl.a(SourceFile:760)
	at cfx.b(SourceFile:107)
	at cfx$$Lambda$1772/194298413.doWork(Unknown Source)
	at cfx$d.doWork(SourceFile:263)
	at cfx.a(SourceFile:219)
	at zs.a(SourceFile:568)
	... 17 more