Merge pull request Automattic#15126 from Automattic/vkarpov15/updateone-null

fix(model): avoid throwing unnecessary error if `updateOne()` returns `null` in `save()`

Author: vkarpov15

lib/model.js

@@ -388,7 +388,11 @@ Model.prototype.$__handleSave = function(options, callback) {
 
     this[modelCollectionSymbol].updateOne(where, update, saveOptions).then(
       ret => {
-        ret.$where = where;
+        if (ret == null) {
+          ret = { $where: where };
+        } else {
+          ret.$where = where;
+        }
         callback(null, ret);
       },
       err => {