core/selinux-access: do not use NULL for %s

core/selinux-access: do not use NULL for %s

../src/core/selinux-access.c: In function ‘mac_selinux_generic_access_check’:
../src/basic/log.h:223:27: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
../src/core/selinux-access.c:235:85: note: format string is defined here
  235 |                         log_warning_errno(errno, "SELinux getcon_raw failed (tclass=%s perm=%s): %m", tclass, permission);
      |                                                                                     ^~

I wonder why nobody ever noticed this.

Fixes #14691 (other issues listed in that ticket have already been fixed).