DAG parser can create task with nulled input jobs

[Mequrel]

task.getInputFiles() or task.getOutputFiles should never return null. It's better to return empty list.

Please remove also null sanity checks from DAGDynamicScheduler

[bryk]

Side comment: there are plenty of sanity checks everywhere. I'd be better if we had unit test instead :)
But the code is already written, so we have somehow to live with this.

I agree that incremental removal of sanity checks and additions of unit tests is good.