return nil if ui cgcontext comes out NULL

Starting in Xcode5 usage of a NULL graphics context starting 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.