GIRAPH-1165: Skip iterating through vertices in supersteps with just …

[majakabiljo]

…global logic

Summary: Some supersteps don't do anything with vertices but just do global worker or master computation or perform aggregation. Not iterating through vertices in these cases can save time.

Test Plan: Ran some jobs with supersteps which only do global computation and verified computation is skipped and we save a few seconds per these supersteps