lib: avoid create a new empty object for optionEnv

Author: alexcfyung

lib/child_process.js

@@ -493,9 +493,9 @@ ObjectDefineProperty(execFile, promisify.custom, {
 });
 
 function copyProcessEnvToEnv(env, name, optionEnv) {
-  optionEnv = optionEnv || {};
   if (process.env[name] &&
-      !ObjectPrototypeHasOwnProperty(optionEnv, name)) {
+      (!optionEnv ||
+       !ObjectPrototypeHasOwnProperty(optionEnv, name))) {
     env[name] = process.env[name];
   }
 }