In ClickUp formula column, is there a way to filter empty data in logical expression?

Question

I've created a formula to return status value of a task being "Late" or "Due". However, the task/row without Start Date should display "-" instead. The formula I wrote unable to return the expected result.

IF(field("Start date") = null,"-", IF(AND(field("Start date")< TODAY(),field("% Complete")=0),"⚠️Late",IF(field("Due date")>= TODAY(),"-","Due")))

FYI, I've alsi tried additional method

= null
= "null"
= 'null'
= 0
= ""
= " "
= "-"

all above not working. Appreciated if anyone can help

Unable to filter empty start date