Fix division by zero error in TableauFicheGlobale

We may want to catch and log instead ? But in that case, who's gonna see the log in the daily cron generation ?

Also: strange thing, but it does not fail in local, only in production (I've ported this code on production to make sure it fixes the issue…), so it may be a difference in the data I've not seen, or it may be that I've not understood something!