types(cursor): indicate that cursor.next() can return null

Fix #14787

test/types/querycursor.test.ts

@@ -21,6 +21,8 @@ Test.find().cursor().
   }).
   then(() => console.log('Done!'));
 
+Test.find().cursor().next().then((doc) => expectType<ITest | null>(doc));
+
 async function gh14374() {
   // `Parent` represents the object as it is stored in MongoDB
   interface Parent {

types/cursor.d.ts

@@ -51,7 +51,7 @@ declare module 'mongoose' {
      * Get the next document from this cursor. Will return `null` when there are
      * no documents left.
      */
-    next(): Promise<DocType>;
+    next(): Promise<DocType | null>;
 
     options: Options;
   }