systemctl: fix editing of units with no fragment

systemctl: fix editing of units with no fragment

"systemctl --user edit --force --full tmp.mount" would crash, when we'd do
basename(NULL). Fix this by creating a new unit or a new override even if
not path is found.

Tested with:
systemctl --user edit --force --full tmp.mount
systemctl --user edit --force tmp.mount
systemctl --user edit foo@.service
systemctl --user edit foo@bar.service
systemctl --user edit --full foo@.service
systemctl --user edit --full foo@bar.service