Null check before calling g.close()

For use in case when clean is called with just the Configuration (prior to test start to cleanup from a previous run).
Call clear with just configuration prior to test starts (and graph creation).

This will clean up any nonsense left over from a previously failed test.
Calling tx.success no longer causes a fail situation.

Not sure why this issue resolved itself, but must have somehow been related to other changes made in prior commits.
Improve efficacy of generator tests.

More assertions around actual creation of edges/vertices.  Some of these tests really weren't even testing anything even with TinkerGraph.
Add todo comments as reminders for considering transactions with generators.