Position of "null" in custom field sort order

We have a custom field in our Zendesk instance called Salesforce Priority. It's a drop-down with values 1, 2, 3, etc. And sometimes this field is not assigned a value. 

When looking at a view that sorts on this field, NO VALUE is sorted as the highest possible value, above 1. I'd like it to be the lowest possible value. I don't know how to get "NO VALUE" onto the sort list. Entering "-" doesn't work, even though "-" is how "NO VALUE" appears in the interface.