FAKEHBASE-24. FakeScanner causes NPE if setStopRow is set on an empty table.

If you set an empty table with a stopRow set, the scanner will get a null pointer exception.

review: https://review.kiji.org/r/1126
issue: https://jira.kiji.org/browse/FAKEHBASE-24
contributed-by: axiak (Michael Axiak, maxiak@hubspot.com)