Pig Null and bag clarification

[#1]

What is an empty bag, empty tuple, and Null?

How to represent empty bag, empty tuple, and Null?

which operations will generate empty tuple, empty bag?

I have this confusion because I do not have clarity when to use datafu.pig.bags.NullToEmptyBag() and EmptyBagToNullFields functions?

Can someone give some example for my understanding

[#2]

Any Input on my clarifications