search: do not panic in query string function

Created by: rvantonder

This asString function is dangerous. It doesn't do exhaustive checking on the states of *searchquerytypes.Value. The field values v.String, v.Regexp, v.Bool could all be nil, or v could be nil, etc. I'm not fixing up all of that, instead, let's just return a string and not panic.

The uses for asString appear luckily safe for now, and I ran into this while changing some parser logic, so we are potentially one small change away from nil panicking without this PR.