I’d agree the best bet (assuming that you do not have anyone who can open your boîte à lettres and forward you the contents) is to phone/email/send carrier pigeon to your Mairie and ask their advice - as noted above the system does allow for “maisons secondaires” so if you have the magic codes you can complete the census online from the UK.
Respect 
Our commune isn’t involved. No families on donkeys here….
Check before you travel!