Fix PaginatedResult.next() usage

PaginatedResult.next() type was updated in ably-js v2 [1] to return null
in case of no page.

[1] ably/ably-js@8bd1706

src/CursorHistory.ts

@@ -76,7 +76,8 @@ export default class CursorHistory {
     pageNo++;
 
     while (pageNo <= paginationLimit && this.positionsMissing(connections) && history.hasNext()) {
-      page = await history.next();
+      // assert result of .next() is non null as we've checked .hasNext() before
+      page = (await history.next())!;
       connections = this.allCursorUpdates(connections, page);
       pageNo++;
     }