Check null before grabbing metadata owning plugin. Fixes BUKKIT-4665

MetadataStoreBase throws a NullPointerException when passed a null value
for setMetaData. The intended behavior is to throw an
IllegalArgumentException. This commit changes the value's null check to
occur before referencing the owning plugin of a value.
Spigot
EMC POM Changes
Water flow not trigger chunk loads

Make BlockFromToEvent default cancel if the flow would trigger a chunk load.
PlayerDeathEvent.getItemsToKeep

Exposes a mutable array on items a player should keep on death
Add a call helper to Event
Precise timings
EntityShootBowEvent consumeArrow API
ItemStack isSimiliar ability to skip durability check
Expose commandmap
Inventory Meta API
Entity Serialization

add method to spawn an entity at a location if its not in any world (invalid)
Remove Invalidated Metadata

On reload, metadata will be cleared
Add force to spawn reasons
Add API for vehicle exit event reason