I propose the next changes for ComboBox element:
Code:
Object.extend(Ext.form.ComboBox.prototype, {
    doForce: function() {
        if(this.getRawValue().length <= 0){
            if (this.allowBlank) this.lastSelectionText = "";
        }
        this.el.dom.value =
            this.lastSelectionText === undefined ? '' : this.lastSelectionText;
        this.applyEmptyText();
    }
});
This changes to allow empty value for ComboBox element when ComboBox.forceSelection == true and ComboBox.allowBlank == true