Fix Tails Cloner: Reset start button, link and labels when last drive removes (#20069)

Ben Westgaterequested to merge
BenWestgate/tails:remove_last_drive_resets_to_the_Install_condition_#20069 into stable

Currently, if the last target USB stick is a Tails USB and it is removed leaving none the start button continues saying "Upgrade" as well as the link being "Manual Upgrade Instructions".

This MR stops on_target_partitions_changed() from returning early in the Device is None case.

When there are no devices and this method is called, it will reset the interface to the more appropriate Install/Backup condition.

Closes #20069 (closed).

Edited by Ben Westgate

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