function OnClientBlur(sender, args) {            
    var textInTheCombo = sender.get_text();
    var item = sender.findItemByText(textInTheCombo);

    //if there is no item with that text
    if (!item) {
        sender.set_text(sender.get_emptyMessage());
    }
}

function OnClientFocus(sender, args) {
    var textInTheCombo = sender.get_text();

    //if there is no item with that text
    if (textInTheCombo == sender.get_emptyMessage()) {
        sender.set_text("");
    }
}