How to check if REST response field not in response

I have a message class define. But sometimes the RelatedItems fields is not returned. 

Parameter %JSONIGNOREINVALIDFIELD As BOOLEAN = 1;

Property RequestedTerm As AH.AHLIB.Custom.Symedical.Message.PostRelationResponse.termsRequested;

Property RelatedItems As list Of AH.AHLIB.Custom.Symedical.Message.PostRelationResponse.RelatedItems;
 

Respnse normally will return someting like this,

on ocassion the RelatedItems are not returned

have response context variable defined as the custom message type

How do I check in a BPL if there is no RelatedItems  in the response?