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

Let me know if any more testing is needed to be done

[] 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)

      |              ^~~~~~~~~~
/home/clint/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)
      |                  ^~~~~~~~

/home/clint/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.

 tools/testing/selftests/filesystems/anon_inode_test.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

index 94c6c81c2..2c4c50500 100644
@@ -42,7 +42,10 @@ TEST(anon_inode_no_exec)
 	fd_context = sys_fsopen("tmpfs", 0);
 	ASSERT_GE(fd_context, 0);
 
-	ASSERT_LT(execveat(fd_context, "", NULL, NULL, AT_EMPTY_PATH), 0);
+	char *const empty_argv[] = {NULL};
+	char *const empty_envp[] = {NULL};
+
+	ASSERT_LT(execveat(fd_context, "", empty_argv, empty_envp, AT_EMPTY_PATH), 0);
 	ASSERT_EQ(errno, EACCES);
 
 	EXPECT_EQ(close(fd_context), 0);

2.43.0