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

Author: vkarpov15

lib/model.js

@@ -387,7 +387,11 @@ 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 => {