Make CacheThrottle work with DummyCache

This is a tiny change.

I happened to discover that the `CacheThrottle` assumes that the cache isn't blown away between a `set` and a `get` a couple lines later. Of course this issue is hard to discover with any practical cache implementation, but the `DummyCache` always returns `None` so the bug is triggered immediately.

This commit makes the `CacheThrottle` work in all cases.

Cheers,

Kenn