Fix: Let types to check null pointer

 * #29 causes compile error on non-object types like System.Enum on Windows.
 * Add null pointer check to Type.
 * Not all types can handle nullptr, so let types to decide how to react to it
 * override Type.toNullCheck to customize 

```javascript
Type.prototype.toNullCheck = function(varname, indent, code) {
	code.push(indent+'if ('+varname+' == nullptr)');
	code.push(indent+'{');
	code.push(indent+'\treturn JSValueMakeNull(ctx);');
	code.push(indent+'}');
}
```