Improve slow combo box performance with large lists

Prevents unnecessary regex testing and option splitting on initial load when no search string has been applied.

- Do not test select options for matches when filter string is empty.
- Prevent splitting every option on empty string, resulting in erroneous
   \<strong> elements in options list. Ex: ["Alabama"](http://bseth99.github.io/jquery-ui-extensions/tests/visual/combobox/value.html) becomes
```html
<strong></strong>A<strong></strong>l<strong></strong>a<strong></strong>s<strong></strong>k<strong></strong>a<strong></strong></a>
```