Revert [NSNull null] to nil in fulfiller block

We may get a "HTTP 200 OK" or other acceptable status codes from a server with no response object.
In such case we cannot set `nil` as an argument to `PMKManifold()` because it takes an `NSArray`

I just update some codes so now when we are creating a `PMKPromise`, we can write something like `fulfiller(PMKManifold([NSNull null], ...))`, and still get `nil` in `somePromise.then(^(NSDictionary *responseObject, ...) {});`.
This makes it easier to check if response object is nil or not by using `if (responseObject)` rather than `if (responseObject == [NSNull null])`.