Upstream has released updates that appears to apply and compile correctly.
This update has not been tested by PaperMC and as with ANY update, please do your own testing
Bukkit Changes:
76fdc1ef SPIGOT-4411: Document use of null for shulker colours
CraftBukkit Changes:
6a0c4e1c SPIGOT-4411: Document use of null for shulker colours
8e520883a2