Hide the toggle icon if the _children is empty

[nghuuphuoc]

Is your feature request related to a problem? Please describe.*

When using the DataTree module, the toggle icon is always shown even the _children is empty.
Here is the jsfiddle:

var tableDataNested = [
    ...
    {name:"James Newman", ..., _children: []},
];

As you see, the last row children is empty, but in reality, it still shows the toggle icon

Describe the solution you'd like

I want the icon to be hidden if the children is empty.

I guess we need to modify the generateControlElement() function to something like this

// DataTree.js

generateControlElement(row, el){
  if(config.children !== false && config.children?.length > 0) { ... }
}