Add put if not null to CompoundTag

[urielsalis]

Adds a helper method that only puts a tag in a compound tag if its not null

The idea is allow easier chaining while adding new elements (as you dont have to do a null check on each one)

While I was at it I also cleaned up the test a bit, let me know if you want me to revert it

[coveralls]

Coverage increased (+0.04%) to 78.246% when pulling 52680eb on urielsalis:add-put-if-not-null into f05f37f on Querz:master.

[Querz]

lgtm