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_get.cpp b/keystore/keystore_get.cpp
index cf67fa4..fec36c4 100644
--- a/keystore/keystore_get.cpp
+++ b/keystore/keystore_get.cpp
@@ -29,7 +29,7 @@
     sp<android::security::IKeystoreService> service =
         interface_cast<android::security::IKeystoreService>(binder);
 
-    if (service == NULL) {
+    if (service == nullptr) {
         return -1;
     }