Pane scopes are created in the user manager, but when the server is not
in a user unit, for example when it was started from a login session's
session-N.scope, sd_pid_get_unit() returned that system unit and it was
set as Before and PartOf of the pane scope. The user manager cannot see
system units, so this only leaves a reference to a missing unit.
Set the dependencies only when sd_pid_get_user_unit() finds a unit.
GitHub issue 5741.
dualfroz