Changes

SPIGOT-6194: Read correct nbt compound into chunk pdc

Previously spigots chunk pdc loading logic would read the entire chunk
nbt compound into the persistent data container of the chunk instead of
just reading the "BukkitValues".

Furthermore this commit also now correctly checks if the nbt compounds
of entities, tile entities and chunks actually have a value for the
"BukkitValues" key, as the previous 'getCompound' call would always
return an instance, the null check was useless. This commit now uses
'get', which returns null if no key exists and then runs an instanceof
check to both validate a non-null instance and an NBTTagCompound
instance.