Create NullOperation

[nkinesis]

• From now on the assumption is that every Producer, when it is instantiated, has a NullOperation by default
• If it needs a subsequent operation, it will receive one (given by the runner)
• Otherwise, the NullOp will remain, will do nothing and then notify observers
• Benefit of doing so: less probability of null pointer exceptions, no need for null checks
• Also: Use Optional instead of returning (and testing!) for null: java.util.Optional

[nkinesis]

We end up trading 1 null check by several this.setOperation((IOperation<E>) new NullOperation());.
Is the trade-off worth it? Maybe we should discuss it next meeting.