kernel: k_timer_init: use NULL when initializing user data

Fixes sparse warning:
<snip>/zephyr/kernel/timer.c:105:28: warning: Using plain integer as NULL pointer
  CC      kernel/timer.o

Change-Id: Ic17a0b976d25079711f10137667148a321c95dbf
Signed-off-by: Maciek Borzecki <maciek.borzecki@gmail.com>