shared/dropin: fix assert for invalid drop-in

Don't try to show top level drop-in for non-existent units or when trying to
instantiate non-instantiated units:

$ systemctl cat nonexistent@.service
Assertion 'name' failed at src/shared/dropin.c:143, function unit_file_find_dirs(). Aborting.
$ systemctl cat systemd-journald@.service
Assertion 'name' failed at src/shared/dropin.c:143, function unit_file_find_dirs(). Aborting.