Resolve #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).

The issue (ticket) explains better about this problem.

## Verification

This was all done by reviewing code. I don't actually all these machines to test. So I guess the verification steps are:

- [x] When you see a get_once, make sure nil is handled.