You case use this spec:

[
  {
    "operation": "shift",
    "spec": {
      "*": "data.&"
    }
 },
  {
    "operation": "modify-overwrite-beta",
    "spec": {
      "*": "=recursivelySquashNulls"
    }
 },
  {
    "operation": "shift",
    "spec": {
      "data": {
        "*": "&"
      }
    }
 }
]

This spec covers all levels of json