Fix assert crash when room map is empty
Fix assert crash when room map is empty

    untech-editor-gui: src/vendor/imgui/imgui_widgets.cpp:721: bool ImGui::InvisibleButton(const char*, const ImVec2&, ImGuiButtonFlags): Assertion `size_arg.x != 0.0f && size_arg.y != 0.0f' failed.

undisbeliever/untech-editor,undisbeliever/untech-editor