perf(schema): make some micro optimizations for casting subdocument paths

Re: #13191

Author: vkarpov15

lib/schema/SubdocumentPath.js

@@ -160,22 +160,22 @@ SubdocumentPath.prototype.cast = function(val, doc, init, priorVal, options) {
   let subdoc;
 
   // Only pull relevant selected paths and pull out the base path
-  const parentSelected = doc && doc.$__ && doc.$__.selected || {};
+  const parentSelected = doc && doc.$__ && doc.$__.selected;
   const path = this.path;
-  const selected = Object.keys(parentSelected).reduce((obj, key) => {
+  const selected = parentSelected == null ? null : Object.keys(parentSelected).reduce((obj, key) => {
     if (key.startsWith(path + '.')) {
       obj = obj || {};
       obj[key.substring(path.length + 1)] = parentSelected[key];
     }
     return obj;
   }, null);
-  options = Object.assign({}, options, { priorDoc: priorVal });
   if (init) {
     subdoc = new Constructor(void 0, selected, doc, false, { defaults: false });
     delete subdoc.$__.defaults;
     subdoc.$init(val);
     applyDefaults(subdoc, selected);
   } else {
+    options = Object.assign({}, options, { priorDoc: priorVal });
     if (Object.keys(val).length === 0) {
       return new Constructor({}, selected, doc, undefined, options);
     }