Add additional constructor callback check for nullptr (#105)

If a JS object constructor is defined natively, and the native callback
returns nullptr, we should return the JS object instance instead of
nullptr. This behavior should match what happens with other JS engines
by default.

Author: ryantrem

Core/Node-API/Source/js_native_api_javascriptcore.cc

@@ -337,7 +337,7 @@ namespace {
         return nullptr;
       }
 
-      return ToJSObject(env, result);
+      return result != nullptr ? ToJSObject(env, result) : instance;
     }
 
     // JSObjectFinalizeCallback