EmptyPlaceHolder causes IndexOutOfBoundsException on MaterialListValueBox#getValue

Selecting the very last item in the list and then calling MaterialListValueBox#getValue causes IndexOutOfBoundsException. This is because MaterialListValueBox#setEmptyPlaceHolder adds a new item into the list but does not add a new value into the array of the underlying model.

MaterialListValueBox#setEmptyPlaceHolder:

    public void setEmptyPlaceHolder(String value) {
        listBox.insertItem(value, 0);

        getOptionElement(0).setDisabled(true);
    }

I did not think this through at all, but maybe adding a null value at the 0-index of the underlying array could work? (Unless someone will beat me to it I shall look into it during this upcoming week.)

[kevzlou7979]

@jzaruba Can you please add a test case for any new features to avoid regressions in the future.

[jzaruba]

@kevzlou7979 I did not consider this one a new feature, rather a bug-fix. (If you referred to #753 then ignore this comment.)