Check preRequirements != null before accessing them

I had a case where `resolution.Before` was null and thus an exception was thrown. I don't really know why, but since DefaultRouteDispatcher also [performs this check](https://github.com/NancyFx/Nancy/blob/5ea5934f7573d80278058b77aa4a0b5595ce0beb/src/Nancy/Routing/DefaultRequestDispatcher.cs#L137), it should be the right thing to do.