You can use a File/Folder Condition to check for the existence/availability of the UNC share. This allows you to redirect the file wherever you want when the destination folder is unavailable.
Note that the use of UNC paths is strongly discouraged as it introduces delays caused by Windows authentication, which occurs each time you access the UNC, as well as potential errors if the folder is not available. Mapped drives are the preferred way of accessing remote shared folders, especially since you can map them automatically through the Startup process.
While mapped drives don’t resolve issues related to the unavailability of the destination, they do resolve latency issues since authentication is only performed once, when the drive is being mapped.