I'm pleased to share that you can now build Flows with 'owner unassigned' in mind.
This week, we released "is blank" and "is not blank" as two new operators for most Flows filter options. In practical terms, you can now set
Owner = is blank
in Flows, which will run for tickets where there are no members currently assigned.
You can find this by navigating to Admin > Flows, selecting a Filter option, and then selecting the operator dropdown menu to the right of the filter option.