GUI: fix empty tree node

src/gui/settings.cpp

@@ -479,6 +479,8 @@ void FurnaceGUI::drawSettings() {
             }
 
             ImGui::TextWrapped("the following values are common (in red, green, blue, alpha order):\n- 24 bits: 8, 8, 8, 0\n- 16 bits: 5, 6, 5, 0\n- 32 bits (with alpha): 8, 8, 8, 8\n- 30 bits (deep): 10, 10, 10, 0");
+          } else {
+            ImGui::Text("nothing to configure");
           }
           ImGui::TreePop();
         }