fix(types): clean up a couple of issues with $add and $ifNull

Fix #12017

Author: vkarpov15

test/types/expressions.test.ts

@@ -86,6 +86,8 @@ const topN: Expression.TopN = {
   }
 };
 
+const d: Expression.Avg = { $avg: { $subtract: [{ $ifNull: ['$end', new Date()] }, '$start'] } };
+
 const dateSubtract1: Expression = {
   $dateSubtract:
   {
@@ -167,6 +169,13 @@ const letExpr: Expression = {
   }
 };
 
+const addWithNull: Expression.Add = {
+  $add: [
+    '$price',
+    { $ifNull: ['$tax', 0] }
+  ]
+}
+
 const toLong: Expression = { $toLong: '$qty' };
 
 const nullExpr: Expression = {

types/expressions.d.ts

@@ -32,7 +32,7 @@ declare module 'mongoose' {
        *
        * @see https://docs.mongodb.com/manual/reference/operator/aggregation/add/#mongodb-expression-exp.-add
        */
-      $add: (NumberExpression | DateExpression)[];
+      $add: Expression[];
     }
 
     export interface Ceil {
@@ -1967,7 +1967,7 @@ declare module 'mongoose' {
        * @version 5.0
        * @see https://docs.mongodb.com/manual/reference/operator/aggregation/avg/#mongodb-expression-exp.-avg
        */
-      $avg: ArrayExpression;
+      $avg: Expression;
     }
 
     export interface Count {