Andy,
I could not reproduce the case where the time is captured only, but to get the data completely from the json response I create a small testcase. In a MF I created a string with the json containing the DevEUI_uplink. Then I created a TopLevel entity object and deserialized the json. This will result in all data from the json being captured in the DataEntity object. The domain model for the test case looks like:
And the MF for the testcase:
Hope this helps you in getting the data you need from the json response.