Use std::optional instead of sentinel values for absent data

src/ui/SelectedBuildingPanel.h

@@ -109,7 +109,7 @@ private:
     ShipLayoutPreview* m_layoutPreview;
     QPushButton*       m_configureLayoutBtn;
 
-    BuildingId     m_singleBuildingId;
+    std::optional<BuildingId> m_singleBuildingId;
     bool         m_singleIsSite = false;  // selected single entity is a construction site
     QPoint       m_splitterTile;
     std::string  m_currentRecipeId;