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_hidl.cpp b/keystore-engine/keystore_backend_hidl.cpp
index 9a84e67..30cf890 100644
--- a/keystore-engine/keystore_backend_hidl.cpp
+++ b/keystore-engine/keystore_backend_hidl.cpp
@@ -33,13 +33,13 @@
 int32_t KeystoreBackendHidl::sign(
         const char *key_id, const uint8_t* in, size_t len, uint8_t** reply,
         size_t* reply_len) {
-    if (key_id == NULL || in == NULL || reply == NULL || reply_len == NULL) {
+    if (key_id == nullptr || in == nullptr || reply == nullptr || reply_len == nullptr) {
         ALOGE("Null pointer argument passed");
         return -1;
     }
 
     sp<IKeystore> service = IKeystore::tryGetService();
-    if (service == NULL) {
+    if (service == nullptr) {
         ALOGE("could not contact keystore HAL");
         return -1;
     }
@@ -63,13 +63,13 @@
 
 int32_t KeystoreBackendHidl::get_pubkey(
         const char *key_id, uint8_t** pubkey, size_t* pubkey_len) {
-    if (key_id == NULL || pubkey == NULL || pubkey_len == NULL) {
+    if (key_id == nullptr || pubkey == nullptr || pubkey_len == nullptr) {
         ALOGE("Null pointer argument passed");
         return -1;
     }
 
     sp<IKeystore> service = IKeystore::tryGetService();
-    if (service == NULL) {
+    if (service == nullptr) {
         ALOGE("could not contact keystore HAL");
         return -1;
     }