No sane state would use nuclear arms against another nuclear armed nation unless threatend in its existence. If a conventional army large enough to overrun Germany would exist and attack Germany, France knows full well that they are next and could not defend themselves conventionally either. But that scenario is rather unlikely.

And beyond that there are a bunch of different game theoretical considerations mostly centering around second strike capabilities to protect against the threat of a sudden unprovoked nuclear strike aiming to decapitate an enemy state, and in all those scenarios France IMHO comes out as having no choice but to extend their nuclear shield to nearly all of Europe.