portabled: refuse queries for empty image name
    
    I took inspiration from pid1:
    bus_unit_find()
      → find_unit()
        → manager_load_unit_from_dbus_path()
          → unit_name_from_dbus_path()
            → !startswith(path, "/org/freedesktop/systemd1/unit/")
              → return -EINVAL
              ←
            ←
          ←
        ← if (r < 0) return 0
      ← 0
    ←
    
    i.e. we return 0 when queried for "/org/freedesktop/systemd1/unit".
    
    Fixes #23445.