Do not format USEC_INFINITY as NULL

systemctl would print 'CPUQuotaPerSecUSec=(null)' for no limit. This
does not look right.

Since USEC_INFINITY is one of the valid values, format_timespan()
could return NULL, and we should wrap every use of it in strna() or
similar. But most callers didn't do that, and it seems more robust to
return a string ("infinity") that makes sense most of the time, even
if in some places the result will not be grammatically correct.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>
udev/hwdb: New Entry for Dell XPS12 9Q33 keyboard

https://bugs.freedesktop.org/show_bug.cgi?id=84437

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>