Fix for NullReferenceException if /auth/logout is entered into the browser directly

Perhaps a trivial thing, but I was experimenting with the authentication support, and went to log out by accessing /auth/logout in the browser directly. The result was a NullReferenceException, and it took me a bit to discover what the problem was. Simple fix is to not redirect if there is no redirect URL available. (In reality, a redirect should always be available or specified - at least as a workaround to this issue.)