Add check for null owner

[cylian914]

In a normal context the check isn't needed as get entity will just return null but its needed for compatibility with the mod MCMT

MCMT replace the byUuid hasmap in net.minecraft.world.level.entity.EntityLookup by a ConcurrentHashMap so it can be access from multiple threads and this hashmap doesn't check if the key is null before trying to get the hashCode() causing a crash