[Quasar Q-Tree] Allow node to be expandable even without children

[timvanstam]

Currently, a node in QTree is only expandable if it has children.
If a node has no children:

The body is always visible

The header is no longer expandable or clickable

Even setting expandable: true has no effect

Feature request:
Make it possible for a node without children to still behave like an expandable node — for example, to show or hide custom content in the body. This would allow consistent UI behavior regardless of whether a node has children.

Is this something that could be supported in a future release?