How can I filter my API call query to use null for a custom field?

I am trying to query tickets where a custom field equals null.

My query looks like this.

https://blahblahblah.freshservice.com/api/v2/tickets/filter?query="salesforce_work_item_id:null"

But it’s not returning anything even though I have tickets with null in that field.

I even tried this

https://blahblahblah.freshservice.com/api/v2/tickets/filter?query="salesforce_work_item_id:''"

Anyone know why I can’t query tickets with null in my custom string field? Am I doing something wrong on my query, or doesn’t the filtering support this?