Clarify that these are existence checks

[PeterJCLaw]

Rather than being emptiness checks (as might otherwise be reasonably assumed given that a Queue is logically a collection).

[PeterJCLaw]

An alternative to this might be to just require that a queue is passed to the CachedState class, rather than letting it be a parameter with a None default. (And then removing these conditional checks.) All current callsites pass a value anyway, so there doesn't seem to be much benefit to this being optional.