Conditional pDraggable and pDroppable

[Axel-Latour]

I'm submitting a ... (check one with "x")

[ ] bug report => Search github for a similar issue or PR before submitting
[x] feature request => Please check if request is not on the roadmap already https://github.com/primefaces/primeng/wiki/Roadmap
[ ] support request => Please do not submit support request here, instead see http://forum.primefaces.org/viewforum.php?f=35

Current behavior
When giving pDraggable or pDroppable directive a null or undefined value, I can drag and drop those elements.

Expected behavior
I would like to disable drag & drop with condition on pDraggable or pDroppable directive, to avoid using *ngIf ...; else ... with the same template, except the activation or deactivation of the drag & drop. If i give a null or undefined value to the directive, the functionality should be disabled.