Cleaned up more cases of NULL -> nullptr

PiperOrigin-RevId: 575226197

upb/test/test_cpp.cc

@@ -57,7 +57,7 @@ TEST(Cpp, Default) {
   upb::Arena arena;
   upb::MessageDefPtr md(upb_test_TestMessage_getmsgdef(defpool.ptr()));
   upb_test_TestMessage* msg = upb_test_TestMessage_new(arena.ptr());
-  size_t size = upb_JsonEncode(msg, md.ptr(), NULL, 0, NULL, 0, NULL);
+  size_t size = upb_JsonEncode(msg, md.ptr(), nullptr, 0, nullptr, 0, nullptr);
   EXPECT_EQ(2, size);  // "{}"
 }
 
@@ -95,10 +95,10 @@ TEST(Cpp, TimestampEncoder) {
     google_protobuf_Timestamp_set_seconds(timestamp_upb, timestamp);
 
     char json[128];
-    size_t size = upb_JsonEncode(timestamp_upb, md.ptr(), NULL, 0, json,
-                                 sizeof(json), NULL);
+    size_t size = upb_JsonEncode(timestamp_upb, md.ptr(), nullptr, 0, json,
+                                 sizeof(json), nullptr);
     bool result = upb_JsonDecode(json, size, timestamp_upb_decoded, md.ptr(),
-                                 NULL, 0, arena.ptr(), NULL);
+                                 nullptr, 0, arena.ptr(), nullptr);
     const int64_t timestamp_decoded =
         google_protobuf_Timestamp_seconds(timestamp_upb_decoded);