General: Make use of std::nullopt where applicable

Allows some implementations to avoid completely zeroing out the internal
buffer of the optional, and instead only set the validity byte within
the structure.

This also makes it consistent how we return empty optionals.

Co-Authored-By: LC <712067+lioncash@users.noreply.github.com>

src/audio_core/hle/decoder.cpp

@@ -55,7 +55,7 @@ std::optional<BinaryResponse> NullDecoder::ProcessRequest(const BinaryRequest& r
         return response;
     default:
         LOG_ERROR(Audio_DSP, "Got unknown binary request: {}", static_cast<u16>(request.cmd));
-        return {};
+        return std::nullopt;
     }
 };
 } // namespace AudioCore::HLE