Fix nextPage typing in requests (#3298)

The fixes the return type of the `nextPage()` method. Before, it would
return `Request<D, E>|void` which is hard to work with, because you
couldn't runtime test for `void` (everything can be void). This change
splits up the definition into two overloads: one without a callback
provided that returns `Request<D, E> | null` and a second for the
callback version that always returns `void`. This also matches the
documentation in the implementation now.

Co-authored-by: Michael Gruenewald <gruenem@amazon.com>

.changes/next-release/bugfix-Paginator-887637f3.json

@@ -0,0 +1,5 @@
+{
+  "type": "bugfix",
+  "category": "Paginator",
+  "description": "Fix nextPage typing in requests"
+}

lib/response.d.ts

@@ -8,7 +8,8 @@ export class Response<D, E> {
 		/**
 		 * Creates a new request for the next page of response data, calling the callback with the page data if a callback is provided.
 		 */
-		nextPage(callback?: (err: E, data: D) => void): Request<D, E>|void;
+		nextPage(): Request<D, E> | null;
+		nextPage(callback: (err: E, data: D) => void): void;
 		/**
 		 * The de-serialized response data from the service.
 		 * Can be null if an error occurred.