man: correct return value of sd_bus_open_with_description

Since https://github.com/systemd/systemd/commit/f4b2933ee7890e5d414ab266d8586f19027a2bd9
if a description is not set, sd_bus_open_with_description returns -ENXIO, but the
documnetation stated that it returned successfully with a NULL string.

(cherry picked from commit 48e5ef14)