Three.js editor UI.Number NaN patch

Fix for when user selects UI.Number element then deletes the value and submits an empty value which causes a NaN value and thus hiding the object from the scene until fixed. This fix replaces the empty/null value from user with 0.

![image](https://f.cloud.github.com/assets/843453/1347189/6225dda0-36ac-11e3-87fc-a196d8e184e4.png)