refactor: quick cleanup to avoid building curPath when not necessary …

…with #14099

lib/helpers/populate/assignVals.js

@@ -144,7 +144,6 @@ module.exports = function assignVals(o) {
 
     const parts = _path.split('.');
     let cur = docs[i];
-    let curPath = parts[0];
     for (let j = 0; j < parts.length - 1; ++j) {
       // If we get to an array with a dotted path, like `arr.foo`, don't set
       // `foo` on the array.
@@ -160,14 +159,14 @@ module.exports = function assignVals(o) {
         // If nothing to set, avoid creating an unnecessary array. Otherwise
         // we'll end up with a single doc in the array with only defaults.
         // See gh-8342, gh-8455
+        const curPath = parts.slice(0, j + 1).join('.');
         const schematype = originalSchema._getSchema(curPath);
         if (valueToSet == null && schematype != null && schematype.$isMongooseArray) {
           break;
         }
         cur[parts[j]] = {};
       }
       cur = cur[parts[j]];
-      curPath += parts[j + 1] ? `.${parts[j + 1]}` : '';
       // If the property in MongoDB is a primitive, we won't be able to populate
       // the nested path, so skip it. See gh-7545
       if (typeof cur !== 'object') {