Task search in workspace with no due date returns null

I just do in my browser https://app.asana.com/api/1.0/workspaces/MYWORKSPACEGID/tasks/search?due_on=null

and I got {“data”:}
but I have hundreds of tasks with empty (unset) due date in my workspace
What’s wrong?

And if you remove the due_on does it return something?

request: https://app.asana.com/api/1.0/workspaces/WORKSPACEGID/tasks/search

result:

{ "errors": [ { "message": "You must specify at least one search filter.", "help": "For more information on API status codes and how to handle them, read the docs on errors: https://asana.com/developers/documentation/getting-started/errors" } ] }

But request

https://app.asana.com/api/1.0/workspaces/WORKSPACEGID/tasks/search?completed=false&opt_fields=due_on

works well. But in this case I should parse tasks with null due_on on my own. It’s not a proper way to use API I suppose.