drm/xe/pxp: Don't use 0 to indicate NULL

Explicitly using NULL is the correct approach.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202502050322.VUBMyUHc-lkp@intel.com/


Fixes: dcdd6b84 ("drm/xe/pxp: Allocate PXP execution resources")
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250204200144.1445800-1-daniele.ceraolospurio@intel.com