sd-bus: do not assert if bus description is not set

sd-bus: do not assert if bus description is not set

The code has an explicit fallback using runtime_scope_to_string(), which
is also documented in the man page. So -ENXIO should only be returned when
the fallback doesn't work, i.e. bus->runtime_scope == -EINVAL.

Fixup for f4b2933ee7890e5d414ab266d8586f19027a2bd9.

This should fix the following errors during boot:
May 18 16:05:37 fedora systemd-update-utmp[263]: Assertion 'bus->description' failed at src/libsystemd/sd-bus/sd-bus.c:4201, function sd_bus_get_description(). Ignoring.
May 18 16:05:38 fedora systemctl[360]: Assertion 'bus->description' failed at src/libsystemd/sd-bus/sd-bus.c:4201, function sd_bus_get_description(). Ignoring.
May 18 16:05:38 fedora systemctl[363]: Assertion 'bus->description' failed at src/libsystemd/sd-bus/sd-bus.c:4201, function sd_bus_get_description(). Ignoring.
May 18 16:05:38 fedora systemd-update-utmp[372]: Assertion 'bus->description' failed at src/libsystemd/sd-bus/sd-bus.c:4201, function sd_bus_get_description(). Ignoring.
May 18 16:05:38 fedora @ystemctl[387]: Assertion 'bus->description' failed at src/libsystemd/sd-bus/sd-bus.c:4201, function sd_bus_get_description(). Ignoring.
May 18 16:05:40 fedora systemd-update-utmp[477]: Assertion 'bus->description' failed at src/libsystemd/sd-bus/sd-bus.c:4201, function sd_bus_get_description(). Ignoring.
May 18 16:05:41 fedora systemd-homed[509]: Assertion 'bus->description' failed at src/libsystemd/sd-bus/sd-bus.c:4201, function sd_bus_get_description(). Ignoring.
May 18 16:05:41 fedora systemd-logind[510]: Assertion 'bus->description' failed at src/libsystemd/sd-bus/sd-bus.c:4201, function sd_bus_get_description(). Ignoring.
May 18 16:05:41 fedora systemd-update-utmp[529]: Assertion 'bus->description' failed at src/libsystemd/sd-bus/sd-bus.c:4201, function sd_bus_get_description(). Ignoring.
May 18 16:05:42 fedora systemd-user-runtime-dir[531]: Assertion 'bus->description' failed at src/libsystemd/sd-bus/sd-bus.c:4201, function sd_bus_get_description(). Ignoring.
May 18 16:05:42 fedora systemctl[542]: Assertion 'bus->description' failed at src/libsystemd/sd-bus/sd-bus.c:4201, function sd_bus_get_description(). Ignoring.
May 18 16:05:42 fedora systemd-hostnamed[556]: Assertion 'bus->description' failed at src/libsystemd/sd-bus/sd-bus.c:4201, function sd_bus_get_description(). Ignoring.