If you go back to the docs, it's explained there: https://bryntum.com/docs/gantt/#Gantt/model/TaskModel#property-convertEmptyParentToLeaf-static
By specifying true, all empty parents will be considered leafs. Can also be assigned a configuration object with with the following Boolean properties to customize the behaviour:
onLoad - Apply the transformation on load to any parents without children (children : [])
onRemove - Apply the transformation when all children have been removed from a parent
TaskModel.convertEmptyParentToLeaf = {
onLoad : true,
onRemove : true
}