Nautilus extension: avoid spamming the logs due to missing method.
Without this change I see a dozen or so warnings in the Journal:
nautilus_menu_provider_get_background_items: assertion 'NAUTILUS_IS_MENU_PROVIDER (provider)' failed
Nautilus upstream developers are going to discuss if/how they want to address this on their side; in the meantime they recommend simply adding a no-op method to silence the logs, so here we go.
FWIW this is tricked in Tails as https://labs.riseup.net/code/issues/13174.