feat(jqLite): return [] for .val() on <select multiple> with no selection

Fixes #14370

BREAKING CHANGE: For the jqLite element representing a select element in
the multiple variant with no options chosen the .val() getter used to return
null and now returns an empty array.

To migrate the code follow the example below:

Before:

HTML:

    <select multiple>
        <option>value 1</option>
        <option>value 2</option>
    </select>

JavaScript:

    var value = $element.val();
    if (value) {
        /* do something */
    }

After:

HTML:

    <select multiple>
        <option>value 1</option>
        <option>value 2</option>
    </select>

JavaScript:

    var value = $element.val();
    if (value.length > 0) {
        /* do something */
    }