Replace std::monostate with std::nullptr_t

This represents the underlying meaning better since we're mostly dealing
with the `data` void pointer argument.

src/common/events.h

@@ -236,7 +236,7 @@ void passthrough_event(boost::asio::local::stream_protocol::socket& socket,
     //      because it was not zeroed out by the host) for an event that should
     //      report some data back?
     const auto response_data = std::visit(
-        overload{[&](auto) -> EventResposnePayload { return std::monostate(); },
+        overload{[&](auto) -> EventResposnePayload { return nullptr; },
                  [&](const AEffect& updated_plugin) -> EventResposnePayload {
                      // This is a bit of a special case! Instead of writing some
                      // return value, we will update values on the native VST
@@ -261,7 +261,7 @@ void passthrough_event(boost::asio::local::stream_protocol::socket& socket,
                      plugin->uniqueID = updated_plugin.uniqueID;
                      plugin->version = updated_plugin.version;
 
-                     return std::monostate();
+                     return nullptr;
                  },
                  [&](WantsChunkBuffer&) -> EventResposnePayload {
                      // In this case the plugin will have written its data
@@ -290,7 +290,7 @@ void passthrough_event(boost::asio::local::stream_protocol::socket& socket,
                      const auto time_info =
                          reinterpret_cast<const VstTimeInfo*>(return_value);
                      if (time_info == nullptr) {
-                         return std::monostate{};
+                         return nullptr;
                      } else {
                          return *time_info;
                      }

src/common/logging.cpp

@@ -204,7 +204,7 @@ void Logger::log_event_response(bool is_dispatch,
 
         std::visit(
             overload{
-                [&](const std::monostate&) {},
+                [&](const std::nullptr_t&) {},
                 [&](const std::string& s) {
                     if (s.size() < 32) {
                         message << ", \"" << s << "\"";

src/common/serialization.h

@@ -295,11 +295,8 @@ struct Event {
  * - A specific struct in response to an event such as `audioMasterGetTime` or
  *   `audioMasterIOChanged`.
  * - An X11 window pointer for the editor window.
- *
- * TODO: Replace `std::monostate` with `std::nullptr_t` as it's more expressive
- *       in what it actually represents.
  */
-using EventResposnePayload = std::variant<std::monostate,
+using EventResposnePayload = std::variant<std::nullptr_t,
                                           std::string,
                                           AEffect,
                                           VstIOProperties,
@@ -310,7 +307,7 @@ template <typename S>
 void serialize(S& s, EventResposnePayload& payload) {
     s.ext(payload,
           bitsery::ext::StdVariant{
-              [](S&, std::monostate&) {},
+              [](S&, std::nullptr_t&) {},
               [](S& s, std::string& string) {
                   // `binary_buffer_size` and not `max_string_length`
                   // because we also use this to send back large chunk

src/wine-host/plugin-bridge.cpp

@@ -302,7 +302,7 @@ class HostCallbackDataConverter : DefaultDataConverter {
                 // Depending on whether the host supported the requested time
                 // information this operations returns either a null pointer or
                 // a pointer to a `VstTimeInfo` object.
-                if (std::holds_alternative<std::monostate>(response.payload)) {
+                if (std::holds_alternative<std::nullptr_t>(response.payload)) {
                     time_info = std::nullopt;
                 } else {
                     time_info = std::get<VstTimeInfo>(response.payload);