fix incorrect reset of populate _docs property

Author: vkarpov15

lib/utils.js

@@ -268,7 +268,7 @@ exports.merge = function merge(to, from, options, path) {
       if (isPOJO(from[key])) {
         to[key] = { ...from[key] };
       } else if (Array.isArray(from[key])) {
-        to[key] = from[key].slice();
+        to[key] = [...from[key]];
       } else {
         to[key] = from[key];
       }
@@ -508,7 +508,7 @@ exports.populate = function populate(path, select, model, match, options, subPop
     if (path instanceof PopulateOptions) {
       // If reusing old populate docs, avoid reusing `_docs` because that may
       // lead to bugs and memory leaks. See gh-11641
-      path._docs = [];
+      path._docs = {};
       path._childDocs = [];
       return [path];
     }