Pass null from excel spreadsheet Datasource into JSON??

Any ideas on how to specify NULL in Excel spreadsheet so it gets transferred to JSON in ReadyAPI, SoapUI NG Pro?

JSON:

{"SBCCommunicationDetails": [{
   "CompassionSBCId": "C0028902042",
   "GlobalPartnerSBCId": ${DataSource#GlobalPartnerSBCId},

   "SourceSystem": "SDL",

...

I want this field to end up looking like this in JSON -  "GlobalPartnerSBCId": NULL,

However, no matter how I try to specify the NULL in the Excel spreadsheet, it will not populate like this.

I either get "GlobalPartnerSBCId": "NULL", or ""

Any ideas?  In the spreadsheet I have tried these values:

NULL, blank, {}, [], 0, "NULL", {NULL}