Null check decoded messages in loadPinnedMessage

`decryptMessage` returns null if it fails to decode the passed message. This value
then got fed into `EventHandler::richBody` which logged a warning and cleared `m_pinnedMessage`.
If we instead retain the value as an `EncryptedEvent`, the UI will pin the encrypted event
placeholder instead of hiding the existence of pins.

src/libneochat/neochatroom.cpp

@@ -1186,7 +1186,10 @@ void NeoChatRoom::loadPinnedMessage()
         connection()->callApi<GetOneRoomEventJob>(id(), mostRecentEventId).then([this](const auto &job) {
             auto event = fromJson<event_ptr_tt<RoomEvent>>(job->jsonData());
             if (auto encEv = eventCast<EncryptedEvent>(event.get())) {
-                event = decryptMessage(*encEv);
+                auto decryptedMessage = decryptMessage(*encEv);
+                if (decryptedMessage) {
+                    event = std::move(decryptedMessage);
+                }
             }
             m_pinnedMessage = EventHandler::richBody(this, event.get());
             Q_EMIT pinnedMessageChanged();