kselftest/anon_inode: replace null pointers with empty arrays

kselftest/anon_inode: replace null pointers with empty arrays

Make use of empty (NULL-terminated) array instead of NULL pointer to
avoid compiler errors while maintaining the behavior of the function
intact

[] Testing:
The diff between before and after of running the kselftest test of the
module shows no regression on system with x86 architecture

[] Error log:
~/Desktop/kernel-dev/linux-v1/tools/testing/selftests/filesystems$ make LLVM=1 W=1
  CC       devpts_pts
  CC       file_stressor
  CC       anon_inode_test
anon_inode_test.c:45:37: warning: null passed to a callee that requires a non-null argument [-Wnonnull]
   45 |         ASSERT_LT(execveat(fd_context, "", NULL, NULL, AT_EMPTY_PATH), 0);
      |                                            ^~~~
/usr/lib/llvm-18/lib/clang/18/include/__stddef_null.h:26:14: note: expanded from macro 'NULL'
   26 | #define NULL ((void*)0)
      |              ^~~~~~~~~~
../Desktop/kernel-dev/linux-v1/tools/testing/selftests/../../../tools/testing/selftests/kselftest_harness.h:535:11: note: expanded from macro 'ASSERT_LT'
  535 |         __EXPECT(expected, #expected, seen, #seen, <, 1)
      |                  ^~~~~~~~
../Desktop/kernel-dev/linux-v1/tools/testing/selftests/../../../tools/testing/selftests/kselftest_harness.h:758:33: note: expanded from macro '__EXPECT'
  758 |         __typeof__(_expected) __exp = (_expected); \
      |                                        ^~~~~~~~~
1 warning generated.

Link: https://lore.kernel.org/r/20251215084900.7590-1-clintbgeorge@gmail.com
Signed-off-by: Clint George <clintbgeorge@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>