[mongoose] fixes lean return type (#40335)

* [mongoose] fixes lean return type

commit c74f860f8b (diff-ec6e6a45ee09ed93f5d81cb304c5a0b3) introduced typings for lean (instead of any). 
I like this approach, but it breaks old code, because type script assume T is undefined in this case. Also it didn't respect find or findOne and always used T directly.

This approach fixes therefore two issues:
- by default (if no type is provided), it is any again (backswards compatiblity)
- Depending on the used functino it either returns the given type or an array of the given type.

* fix(lean): it can also return null

types/mongoose/index.d.ts

@@ -2000,7 +2000,7 @@ declare module "mongoose" {
      * getters/setters or other Mongoose magic applied.
      * @param {Boolean|Object} bool defaults to true
      */
-    lean<T>(bool?: boolean | object): Query<T> & QueryHelpers;
+    lean<P = any>(bool?: boolean | object): Query<T extends Array<any> ? P[] : (P | null)> & QueryHelpers;
 
     /** Specifies the maximum number of documents the query will return. Cannot be used with distinct() */
     limit(val: number): this;