GetParams should not fail when `name` is null

`Null` is a legal parameter name when calling `Request.QueryString[null]` - `req.GetParam(null)` works correctly when nameless parameters are present (for example `/some_route/?edit`) but otherwise fails as the control falls through to `TryGetValue` calls that do not support `null` keys.

This commit fixes the problem by bailing out before `TryGetValue` calls when `name` parameter is `null`.