[json_data] empty children array not rendered as leaf

[bmesuere]

When using the json_data plugin, a node with an empty array of children is not rendered as a leaf.

[vakata]

Just do not pass anything (or a "falsy" value) for "children".

Empty array won't work - older versions of jstree used empty arrays to mark a node as having children, that are not yet loaded, this is why those nodes are shown as closed.