Groovy - Asserting that all JSON response elements have content

I have a test scenario with hundreds of response elements and I am trying to script an assertion to ensure that none of the elements returned as null. There must be an easy way to use a wildcard to search the entire response body for a null field, but I can't figure out how to do it. Can anyone suggest a better solution to ensure no nulls were returned?

Response snippet:
{
"errors" : [
],
"transactionId" : "123456789",
"dateTime" : "2017-12-08T22:07:43.099Z",
"observed" : {
"date" : "2017-12-08",
"obs" : 39,
"conditions" : {
"text" : "Clear",
"code" : 85
}
},
etc.....

And the code I have so far:
import com.parasoft.api.*
import groovy.json.JsonSlurper
import java.util.*
import soaptest.api.*

void extractAndAssertResponse(Map input, ScriptingContext context) {
def slurper = new JsonSlurper()
String responseBody = input.get(SOAPUtil.XML_RESPONSE)
Application.showMessage("RESPONSE_BODY: " + responseBody)

    def json = slurper.parseText(responseBody)
    assert json.transactionId != null
    assert json.dateTime != null
    assert json.observed.date != null

}