Depends on D20475. Ref T13272. Currently, if you JX.Request with data like {x: null}, we submit that as ?x=null, i.e. as though null was the string "null".
This is weird and almost certainly never intended/desiarable. In particular, it causes a bug where panels embedded inside tab panels are incorrectly draggable.
It's possible this breaks something which relied on the buggy behavior, but that seems unlikely.