When using HTML5 classList API, do not allow empty strings or white-spac...

...e only strings else it throws errors.

@cpojer @arian 

per my comments here: 

https://github.com/mootools/mootools-core/commit/bd6ac113600fa508017c29237cf03c36171e9603#commitcomment-4646572

This commit just ensures you don't get errors if empty strings are passed to addClass/removeClass.