Before the 'Perform Find' script step, set Capture Error On. Then use Get (FoundCount) to see if the Find returns any records. Then use an If script step to either show the found records or create a new record. I would suggest you offer the user a dialogue to confirm they really do want to create a new record (as opposed to re-define their Find criteria).
I agree with showing a custom dialog to confirm adding a new record. Scanners sometimes fail to completely scan a bar code and in such a case, you would not want a new record automatically created.