Filter instances by variable-value 'empty string'

I would like to filter process instance in the runtime view in the cockpit.
I added a filter by selecting a task (which is actually paused). Now I would like to add another filter based on the value of a string-variable. My problem is, that I need to entry ‘not empty’ or (better) empty, I can’t do that, because there is no way to enter an empty string (camunda changes this into >>??<< meaning 'nothing entered here ;-)).

Is there any documentation about how to use these filters (like, =, wildcards, nulls etc)? I believe that something changed there and would like to know ‘howto’ from first-hand.

Hi @shp,

it is unfortunately not possible to filter by empty string variables for now.
Kindly feel free to create a bug issue.