CSS: Skip falsy values in `addClass( array )`, compress code

CSS: Skip falsy values in `addClass( array )`, compress code

This change makes jQuery skip falsy values in `addClass( array )`
& `removeClass( array )` instead of stopping iteration when the first falsy
value is detected. This makes code like:
```js
elem.addClass( [ "a", "", "b" ] );
```
add both the `a` & `b` classes.

The code was also optimized for size a bit so it doesn't increase the
minified gzipped size.

Fixes gh-4998
Closes gh-5003

(partially cherry picked from commit a338b407f2479f82df40635055effc163835183f)