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).
- Switch from ECJ to IDEA (with potential for kotlin)
- Can pull SRG mappings from FG, reducing amount of manual mappings (basically just class mappings and fixing errors in SRG mappings)
