Modernize codebase by replacing NULL with nullptr

Fixes -Wzero-as-null-pointer-constant warning.

Test: m
Bug: 68236239
Change-Id: I41cd58617d6df6de7942a541fb6dc9519c70bef0
diff --git a/keystore/permissions.cpp b/keystore/permissions.cpp
index 1ba91d9..40be938 100644
--- a/keystore/permissions.cpp
+++ b/keystore/permissions.cpp
@@ -119,7 +119,7 @@
 
 static bool keystore_selinux_check_access(uid_t uid, perm_t perm, pid_t spid) {
     audit_data ad;
-    char* sctx = NULL;
+    char* sctx = nullptr;
     const char* selinux_class = "keystore_key";
     const char* str_perm = get_perm_label(perm);