How to return blank rather than "null"?

I'm generating an array of phone number like the following:

{
"phones": [
          {
            "phoneNumber": "694-440-4539",
            "phoneType": "FAX",
            "extension": "1234"
          },
          {
            "phoneNumber": "412-450-4989",
            "phoneType": "FAX",
            "extension": null
          }
        ]
}

I've set the field to generate blanks 70% of the time; however, rather than being blank, I'm getting 'null'. How do I fix that?