fix: avoid throwing error if saveOptions undefined when invalidating subdoc cache

Fix Automattic#15062

Author: vkarpov15

lib/plugins/saveSubdocs.js

@@ -27,8 +27,10 @@ module.exports = function saveSubdocs(schema) {
         cb(err);
       });
     }, function(error) {
-      // Bust subdocs cache because subdoc pre hooks can add new subdocuments
-      _this.$__.saveOptions.__subdocs = null;
+      // Invalidate subdocs cache because subdoc pre hooks can add new subdocuments
+      if (_this.$__.saveOptions) {
+        _this.$__.saveOptions.__subdocs = null;
+      }
       if (error) {
         return _this.$__schema.s.hooks.execPost('save:error', _this, [_this], { error: error }, function(error) {
           next(error);