Here is the input JSON that I will put into the JOLT transformation processor in NiFi.

  "guests" : {
    "amy" : "amy",
    "theresa" : [ "Chef" ],
    "jim" : [ "Bear Specialist" ],
    "john" : [ "Tour Guide" ],
    "heather" : [ "" ],
    "brad" : [ "" ],
    "thomas" : [ "" ],
    "jane" : [ "" ]
  },
  "dataSchema" : "dataSchema",
  "data" : {
    "key1" : "value1",
    "key2" : {
      "key3" : "value3",
    },
    "label" : "dummyLabel",
    "campingEquipment" : {
      "someKey" : "someValue"
    }
    },
    "mountainRange" : {
      "elevation" : "8000",
      "temperature" : "50"
    }
}

And my expected output JSON is:

{
  "guests" : {
    "amy" : "amy",
    "theresa" : [ "Chef" ],
    "jim" : [ "Bear Specialist" ],
    "john" : [ "Tour Guide" ]
  },
  "dataSchema" : "dataSchema",
  "data" : {
    "key1" : "value1",
    "key2" : {
      "key3" : "value3",
    },
    "label" : "dummyLabel",
    "campingEquipment" : {
      "someKey" : "someValue"
    }
    },
    "mountainRange" : {
      "elevation" : "8000",
      "temperature" : "50"
    }
}

Notice that these fields were removed:

    "heather" : [ "" ],
    "brad" : [ "" ],
    "thomas" : [ "" ],
    "jane" : [ "" ]

Since they are empty strings in the arrays I do not want to include them in the resulting JSON object.

How would I construct a JOLT spec for this?

I tried this spec to remove the empty arrays.

[
  {
    "operation": "modify-overwrite-beta",
    "spec": {
      "guests": {
        "theresa": "=recursivelySquashNulls",
        "jim": "=recursivelySquashNulls",
        "john": "=recursivelySquashNulls",
        "heather": "=recursivelySquashNulls",
        "brad": "=recursivelySquashNulls",
        "thomas": "=recursivelySquashNulls",
        "jane": "=recursivelySquashNulls"
      }
    }
  },
  {
    "operation": "remove",
    "spec": {
      "guests": {
        "theresa": "",
        "jim": "",
        "john": "",
        "heather": "",
        "brad": "",
        "thomas": "",
        "jane": ""
      }
    }
  }
]

This did not work

and simply returned:

  "guests" : {
    "amy" : "amy"
  },

It removed all the fields with empty arrays.