Page "President of France" Paragraph 78

If there is no acting president of the senate, the powers of the president of the republic are exercised by the " Gouvernement ", meaning the Cabinet.

This has been interpreted by some constitutional academics as meaning first the Prime Minister and, if he is himself not able to act, the members of the cabinet in the order of the list of the decree that nominated them.

This is in fact unlikely to happen, because if the president of the Senate is not able to act, the Senate will normally name a new president of the Senate, that will act as President of the Republic.