I don't think it is possible inside the User Task to do something like "if no users meet the role, then do this alternate process".
I was thinking this could work with Signal/Catch events or Error End/Start events in your process, but upon examining those closely, I'm not really seeing how that could work.
A solution comes to mind, but is rather complex. This would require setting up a Stored Procedure in the Forms database that reviews your teams/roles/users tables and gets a count of how many users have the specific role, this value would be populated into a variable in your form process (either through a data lookup inside a form, or through a workflow called from your process). Then in the process, a gateway would determine if the value in that field is zero or higher. If higher, the process continues to the user task, if zero, the process skips the user task and picks up at a later stage. The part about doing a stored procedure could be skipped if using the workflow option, since you could just query the forms db for the info you need, whereas doing that from a form would need the stored procedure to get what you need.