lib: kprintf: allow printing null as strings
lib: kprintf: allow printing null as strings

If we try to print a NULL pointer with a %s format, we will get a nice Oops.
Lets print "(null)" instead.

Signed-off-by: Fabien Parent <2a93cde040adee823a667eed2882d05f12a84573@gmail.com>

mtitinger/phabos,mtitinger/phabos