Prevent null keys and values and EndTag values in CompoundTag and ListTag

[HoldYourWaffle]

Title says it all :)

NBT doesn't have a concept of null, and CompoundTag is clearly not designed for null keys or values. In the interest of failing fast it'd be good to reject any rogue null's upfront.

Edit: I also added a check to prevent EndTag values in CompoundTag. ListTag's updateType already took care of this.

[Querz]

Since this PR also addresses the array tags:

ListTag should have a check in the ListTag(List<Tag>, byte) constructor, because its underlying value should not be null.

[HoldYourWaffle]

Good catch, I've added some checks in ListTag :)

[HoldYourWaffle]

Whoops, github does not like renaming branches...