Use DefaultedList for equipment tracking

Because on 1.11+ a `null` item stack is not something that should exist.
This then also allows us to compare item stacks by value rather than only by
reference, potentially saving a few redundant entity equipment update
packets (but more importantly, also allows us to store a copy instead of the
original, which will be important for the next commit).

versions/1.11.2/mapping.txt

@@ -1 +1,2 @@
 net.minecraft.item.ItemStack EMPTY field_190927_a
+net.minecraft.util.NonNullList withSize() func_191197_a()