Modernize codebase by replacing NULL with nullptr

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

Test: m
Bug: 68236239
Change-Id: I226a0599db4f7c3557e55cade7869d00bd314949
diff --git a/tests/dns_responder/dns_responder.cpp b/tests/dns_responder/dns_responder.cpp
index 580b92e..1c18503 100644
--- a/tests/dns_responder/dns_responder.cpp
+++ b/tests/dns_responder/dns_responder.cpp
@@ -501,7 +501,7 @@
                                   unsigned* qdcount, unsigned* ancount,
                                   unsigned* nscount, unsigned* arcount) {
     if (buffer + sizeof(Header) > buffer_end)
-        return 0;
+        return nullptr;
     const auto& header = *reinterpret_cast<const Header*>(buffer);
     // bytes 0-1
     id = ntohs(header.id);
diff --git a/tests/dns_responder/dns_tls_frontend.cpp b/tests/dns_responder/dns_tls_frontend.cpp
index 07c7fee..8c49254 100644
--- a/tests/dns_responder/dns_tls_frontend.cpp
+++ b/tests/dns_responder/dns_tls_frontend.cpp
@@ -38,7 +38,7 @@
 
 // Copied from DnsTlsTransport.
 bool getSPKIDigest(const X509* cert, std::vector<uint8_t>* out) {
-    int spki_len = i2d_X509_PUBKEY(X509_get_X509_PUBKEY(cert), NULL);
+    int spki_len = i2d_X509_PUBKEY(X509_get_X509_PUBKEY(cert), nullptr);
     unsigned char spki[spki_len];
     unsigned char* temp = spki;
     if (spki_len != i2d_X509_PUBKEY(X509_get_X509_PUBKEY(cert), &temp)) {
@@ -47,7 +47,7 @@
     }
     out->resize(test::SHA256_SIZE);
     unsigned int digest_len = 0;
-    int ret = EVP_Digest(spki, spki_len, out->data(), &digest_len, EVP_sha256(), NULL);
+    int ret = EVP_Digest(spki, spki_len, out->data(), &digest_len, EVP_sha256(), nullptr);
     if (ret != 1) {
         ALOGE("Server cert digest extraction failed");
         return false;
@@ -92,7 +92,7 @@
         ALOGE("RSA_new failed");
         return nullptr;
     }
-    if (!RSA_generate_key_ex(rsa.get(), 2048, e.get(), NULL)) {
+    if (!RSA_generate_key_ex(rsa.get(), 2048, e.get(), nullptr)) {
         ALOGE("RSA_generate_key_ex failed");
         return nullptr;
     }
diff --git a/tests/tun_interface.cpp b/tests/tun_interface.cpp
index 3d4ab23..34896c7 100644
--- a/tests/tun_interface.cpp
+++ b/tests/tun_interface.cpp
@@ -54,8 +54,8 @@
     sockaddr_in6 src6 = { .sin6_family = AF_INET6, .sin6_addr = mSrcAddr, };
     sockaddr_in6 dst6 = { .sin6_family = AF_INET6, .sin6_addr = mDstAddr, };
     int flags = NI_NUMERICHOST;
-    if (getnameinfo((sockaddr *) &src6, sizeof(src6), srcStr, sizeof(srcStr), NULL, 0, flags) ||
-        getnameinfo((sockaddr *) &dst6, sizeof(dst6), dstStr, sizeof(dstStr), NULL, 0, flags)) {
+    if (getnameinfo((sockaddr *) &src6, sizeof(src6), srcStr, sizeof(srcStr), nullptr, 0, flags) ||
+        getnameinfo((sockaddr *) &dst6, sizeof(dst6), dstStr, sizeof(dstStr), nullptr, 0, flags)) {
         return -EINVAL;
     }