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
a338b407f2