Please try to override the
keyHandler.enter function for your combobox editor.
"editor":{
"type":"combobox",
"options":{
//...
keyHandler:$.extend(true,{},$.fn.combobox.defaults.keyHandler,{
enter:function(e){
var s = $(this).combobox('getText');
$.fn.combobox.defaults.keyHandler.enter.call(this,e);
if (!$(this).combobox('getText')){
$(this).combobox('setValue', s);
}
}
})
}
}