fixed render on window resizing for nil HighlightTheme or StyleName

I made a fix on ```renderIfPreferencesChanged``` to avoid marking as changed if the ```highlightingTheme``` or the ```rendererStyleName``` were nil. This fix is needed because:

```
// If the result of the [d renderererHighlightThemeName:self] return nil
// then the message isEqualToString: over nil will return NO
// the result of this expression will be YES independently of the value of self.highlightThemeName
// so if self.highlightTheme is also nil will return YES being both strings equals to nil
![[d renderererHighlightThemeName:self] isEqualToString:self.highlightThemeName]
```

I made a direct comparison between the stored value and the new one to avoid mark as change if they are both nil.