Modernize codebase by replacing NULL with nullptr

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

Test: m
Bug: 68236239
Change-Id: I226a0599db4f7c3557e55cade7869d00bd314949
diff --git a/libnetdutils/Syscalls.cpp b/libnetdutils/Syscalls.cpp
index 9a05e3b..ac5fef2 100644
--- a/libnetdutils/Syscalls.cpp
+++ b/libnetdutils/Syscalls.cpp
@@ -181,7 +181,7 @@
 
     StatusOr<UniqueFile> fopen(const std::string& path, const std::string& mode) const override {
         UniqueFile file(::fopen(path.c_str(), mode.c_str()));
-        if (file == NULL) {
+        if (file == nullptr) {
             return statusFromErrno(errno, "fopen(\"" + path + "\", \"" + mode + "\") failed");
         }
         return file;