Why does the default empty string value of myBackingBean.yearStr becomes null after the page is loaded?
My Environment:
I am using RichFaces 3.3 with JBOSS 6, and Seam 2.1.2
Its working well without seam

<h:selectOneMenu id="year" value="#{myBackingBean.yearStr}">
        <a4j:support focus="year" event="onchange" reRender="myDataTable" ignoreDupResponses="true" />
        <f:selectItems value="#{comboBoxUtil.yearsValues}" />
</h:selectOneMenu>

comboBoxUtil code:
public List<SelectItem> getYearsValues() { 
        List<SelectItem> items = new LinkedList<SelectItem>();               
                       
        items.add(new SelectItem("", ""); //Most important line:

        for (int i = 1; i < 10; i++) {
                items.add(new SelectItem("0" + i, EMPTY + i));
        }


        for (int i = 10; i < 100; i++) {
                items.add(new SelectItem(EMPTY + i, EMPTY + i));
        }
        return items;
}

The declaration of yearStr in the backing bean (there is also getter and setter):
private String yearStr = "";