PixelPaint: Only clear `LayerListWidget` if there are no editors left

Previously, we would clear it if there was still an editor open. This
was not obvious because it was only visible when an inactive tab was
closed, since closing an active tab would trigger an editor change
which would re-fill the layers widget.

Userland/Applications/PixelPaint/MainWidget.cpp

@@ -68,7 +68,7 @@ MainWidget::MainWidget()
         if (image_editor.request_close()) {
             m_tab_widget->deferred_invoke([&] {
                 m_tab_widget->remove_tab(image_editor);
-                if (m_tab_widget->children().size() == 1) {
+                if (m_tab_widget->children().size() == 0) {
                     m_layer_list_widget->set_image(nullptr);
                     m_layer_properties_widget->set_layer(nullptr);
                 }