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-engine/keystore_backend_binder.cpp b/keystore-engine/keystore_backend_binder.cpp
index f9e7be0..79b0ec3 100644
--- a/keystore-engine/keystore_backend_binder.cpp
+++ b/keystore-engine/keystore_backend_binder.cpp
@@ -42,7 +42,7 @@
     sp<IBinder> binder = sm->getService(String16(keystore_service_name));
     sp<IKeystoreService> service = interface_cast<IKeystoreService>(binder);
 
-    if (service == NULL) {
+    if (service == nullptr) {
         ALOGE("could not contact keystore");
         return -1;
     }
@@ -66,7 +66,7 @@
     sp<IBinder> binder = sm->getService(String16(keystore_service_name));
     sp<IKeystoreService> service = interface_cast<IKeystoreService>(binder);
 
-    if (service == NULL) {
+    if (service == nullptr) {
         ALOGE("could not contact keystore");
         return -1;
     }