Fix: native object to JS object should return JS null when pointer is null

XX_ToJSValue function should check instance and return JS null object, instead of returning new JS object that contains null pointer. might be related to https://github.com/appcelerator/hyperloop-ios/issues/13 .

```c++
/**
 * called to convert an NSObject * to a JSValueRef
 */
EXPORTAPI JSValueRef NSObject_ToJSValue(JSContextRef ctx, NSObject * instance, JSValueRef *exception)
{
	if (instance == nullptr)
	{
		return JSValueMakeNull(ctx);
	}
//...
	return JSObjectMake(ctx, RegisterClass(), po);
}
```