Non-empty cells returned as empty will skip spawns

[c00ni]

Whether it's because the account needed a captcha or for whatever other reason (including this mysterious account state where it doesn't need a captcha but is returning empties), especially in iscan, an empty cell should be re-added to the queue.
Accounts that return known-not-empty-but-reported-empty cells should also be (ideally, in my opinion) put to sleep for a while or logged out. I say ideally put to sleep because you may end up running out of scan accounts and I believe accounts eventually do recover.