How to use trigger-state to constrain based upon null property?

I’m working on a lighting automation that should behave differently if someone manually turns on the light switch.

The Zwave switch reports a null value in the data.new_state.context.user_id property, which seems to be the only property that’s different based upon manually switching the dimmer.

Knowing that, if I want to trigger a flow based upon the user_id being null, I’m unsure of how to do that with the trigger-state constraint.

I can add a constraint for This Entity with the Property new_state.context.user_id but I’m stuck on what to put in the “Is” field. Leaving it empty doesn’t seem to == null and neither does adding empty quotes. How do I test for an empty property in the trigger-state node?

I’ve managed to succeed with the state_changed node combined with a switch node, but using a single node would be nice too.

If you do a regex the null value should be typecast to a string which you can compare to ^null$