return nil if ui cgcontext comes out NULL

Starting in Xcode5 usage of a NULL graphics context started kicking up the following warning:

```
CGContextDrawImage: invalid context 0x0. This is a serious error.
This application, or a library it uses, is using an invalid context 
and is thereby contributing to an overall degradation of system
stability and reliability. This notice is a courtesy: please fix this
problem. It will become a fatal error in an upcoming update.
```

If ```resizedImageToSize:``` is passed ```CGRectZero``` then you end up with a NULL graphics context. This fix causes an early bailout to prevent use of a NULL graphics context.