Resove #4408 - bad uncaught nil get_once

Sometimes get_once can return nil, and if that's not handled, it can cause a backtrace. If there is a backtrace, it means one host that causes the backtrace can cause the entire scan to stop (and that you will probably never be able to scan the whole network with it).

## Verification

This was all done by reviewing code. I don't actually all these machines to test. So I guess the verification steps are:

- [ ] When you see a get_once, make sure nil is handled.