CraftBukkit removed their implementation that caused this issue,
switching to Mojang's implementation which doesn't appear to share it. I
already removed the important bit in the last upstream merge, this is
just unused and unnecessary now. So we remove it.
Spigot has patched this issue inside MapIcon, meaning that we no longer need to maintain this patch; Spigots patch also fixes#668 in that it will verify the length of the array, as well as protect against a negative type value being fetched from the array. Only real change is that Spigots patch returns a MapIcon.Type.PLAYER, instead of the RED_MARKER as originally PR'd by Aikar.
Every call to .isEmpty() made a horribly wasteful map lookup just to get the
reference to the Air Item for checking.
We will now cache a copy of that item
