Fix `RetrofitResultsProducer` Error

[SikandarEjaz]

Running the following queries:

{
    "use": "ca.concordia.encs.citydata.producers.RetrofitResultsProducer",
    "withParams": [
        {
            "name": "buildingIds",
            "value": [ "05071763", "05191025", "01027176" ]
        }
    ],
    "apply": [ ]
}

and

{
    "use": "ca.concordia.encs.citydata.producers.RetrofitResultsProducer",
    "withParams": [
        {
            "name": "buildingIds",
            "value": [ "01002777", "01002778" ]
        }
    ],
    "apply": [
        {
            "name": "ca.concordia.encs.citydata.operations.JsonReadOperation",
            "withParams": [
                {
                    "name": "path",
                    "value": "results.current status[0].costs.global_capital_costs.B2010_opaque_walls"
                }
            ]
        }
    ]
} 

Gives the following error:

[{"result":"[Cannot invoke \"com.google.gson.JsonElement.getAsString()\" because the return value of \"com.google.gson.JsonObject.get(String)\" is null, caused by: null]"}]