Related to #4174, #4717.
As it stands, `JsonList` throws an NPE on `jsonarray.iterator()` when
the reader given is at EOF. This means there is an unintelligible
message shown to the player:
```
[16:44:31 ERROR]: Encountered an unexpected exception
java.lang.NullPointerException: Cannot invoke "com.google.gson.JsonArray.iterator()" because "jsonarray" is null
at net.minecraft.server.v1_16_R2.JsonList.load(JsonList.java:192) ~[patched_1.16.3.jar:git-Paper-248]
at net.minecraft.server.v1_16_R2.DedicatedPlayerList.y(SourceFile:95) ~[patched_1.16.3.jar:git-Paper-248]
...
```
This is clearly not understandable to the untrained eye: what is
`jsonarray`? What is `DedicatedPlayerList.y`?
This rather does a proper exception message:
```
[16:42:59 ERROR]: Encountered an unexpected exception
java.lang.NullPointerException: The file "banned-players.json" is either empty or corrupt
at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:787) ~[paper-1.16.3.jar:git-Paper-"8e6d90c2d"]
at net.minecraft.server.v1_16_R2.JsonList.load(JsonList.java:190) ~[paper-1.16.3.jar:git-Paper-"8e6d90c2d"]
at net.minecraft.server.v1_16_R2.DedicatedPlayerList.y(SourceFile:95) ~[paper-1.16.3.jar:git-Paper-"8e6d90c2d"]
...
```
It is still an exception, which server owners may or may not understand,
but the message now says exactly what went wrong, and who would've
known, now we know what `DedicatedPlayerList.y` is, and we know what
file was broken.
This is more of a quality of life change, but it is certainly useful to
the server owners that come into `#paper-help` and the issue tracker
(#4174) to ask for help with unintelligible messages we need to open a
patched Paper project to make any sense out of.
Check state of jsonarray instead of checkNotNull
Co-authored-by: Riley Park <riley.park@meino.net>
