Under some situations, such as when there are no control points, ControlPointInfo will return to us a newly-constructed controlpoint with every call to SamplePointAt, which will fail on this key lookup.
Between this and overriding GetHashCode, I think this is the more proper fix for this.
This is a temporary solution for now that uses DrawableHitObject.SampleNamespace for the override. We will not want to do this going forward, and instead have the rulesets add their custom resource stores to the games', but that requires deciding where/when to apply/remove such resource stores, and is probably left to skinning.
