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/keystore_cli_v2.cpp b/keystore/keystore_cli_v2.cpp
index bf04209..157417f 100644
--- a/keystore/keystore_cli_v2.cpp
+++ b/keystore/keystore_cli_v2.cpp
@@ -496,7 +496,7 @@
     sp<android::IServiceManager> sm = android::defaultServiceManager();
     sp<android::IBinder> binder = sm->getService(String16("android.security.keystore"));
     sp<IKeystoreService> service = android::interface_cast<IKeystoreService>(binder);
-    if (service == NULL) {
+    if (service == nullptr) {
         printf("error: could not connect to keystore service.\n");
         return 1;
     }