An empty node shouldn't has the 'closed' state. You can set it to a file node with the folder icon. Please refer to the code below.
<style type="text/css">
.node-empty{
background: url('https://www.jeasyui.com/easyui/themes/default/images/tree_icons.png') no-repeat -208px 0;
}
</style>
"children": [{
"id": 11,
"text": "Sub folder 11",
"state": "closed",
"children": [{
"id": 111,
"text": "Empty Sub folder to id 11",
"state": "open",
"iconCls": "node-empty"
}]
},{