gnrc_pktbuf_static: unify NULL pointer output for stats
gnrc_pktbuf_static: unify NULL pointer output for stats

Different platforms evaluate `printf()` for NULL pointers differently,
resulting tests checking for a certain output to fail. This unifies that
(debug) output for the static packet buffer statistics.

x3ro/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,mtausig/RIOT,authmillenon/RIOT,authmillenon/RIOT,josephnoir/RIOT,kaspar030/RIOT,smlng/RIOT,yogo1212/RIOT,toonst/RIOT,basilfx/RIOT,aeneby/RIOT,x3ro/RIOT,basilfx/RIOT,miri64/RIOT,authmillenon/RIOT,ant9000/RIOT,josephnoir/RIOT,miri64/RIOT,OlegHahm/RIOT,kYc0o/RIOT,OlegHahm/RIOT,mtausig/RIOT,OTAkeys/RIOT,josephnoir/RIOT,toonst/RIOT,kaspar030/RIOT,basilfx/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,toonst/RIOT,x3ro/RIOT,kYc0o/RIOT,miri64/RIOT,yogo1212/RIOT,kYc0o/RIOT,x3ro/RIOT,OlegHahm/RIOT,basilfx/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,ant9000/RIOT,josephnoir/RIOT,josephnoir/RIOT,yogo1212/RIOT,OlegHahm/RIOT,jasonatran/RIOT,yogo1212/RIOT,aeneby/RIOT,aeneby/RIOT,kaspar030/RIOT,smlng/RIOT,mtausig/RIOT,x3ro/RIOT,yogo1212/RIOT,smlng/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,basilfx/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,jasonatran/RIOT,mtausig/RIOT,ant9000/RIOT,authmillenon/RIOT,yogo1212/RIOT,mtausig/RIOT,smlng/RIOT,toonst/RIOT,ant9000/RIOT,miri64/RIOT,jasonatran/RIOT,aeneby/RIOT,kaspar030/RIOT,ant9000/RIOT,authmillenon/RIOT,authmillenon/RIOT,toonst/RIOT,miri64/RIOT,aeneby/RIOT,jasonatran/RIOT,smlng/RIOT