shared/install: fix DefaultInstance expansion in %n, %N

shared/install: fix DefaultInstance expansion in %n, %N

We should substitute DefaultInstance if the instance is not specified.

Test case:
[Install]
DefaultInstance=bond1
WantedBy= foobar-n-%n.device
WantedBy= foobar-N-%N.device

$ systemctl --root=/ enable testing4@.service
Created symlink /etc/systemd/system/foobar-n-testing4@bond1.service.device.wants/testing4@bond1.service → /etc/systemd/system/testing4@.service.
Created symlink /etc/systemd/system/foobar-N-testing4@bond1.device.wants/testing4@bond1.service → /etc/systemd/system/testing4@.service.

(before, the symlink would be created with empty %n, %N parts).