It is not illegal to sell a vehicle without a spare wheel and, in fact, it is reportedly more and more common for new vehicles (including motorhomes) to be sold without a spare (roue de secours).
They are often ‘replaced’ by a simple emergency puncture repair kit (kit anti-crevaison), aimed at a temporary repair of minor punctures, typically involving a sealant product squirted into the tyre and then the use of a small compresser unit. Alternatively some cars are sold with a light-weight (thin) tyre known as une galette, designed just to get you to a garage (driving at a slow, prudent speed).
This is said to be because punctures are a fairly rare event with today’s improved tyres, as well as possibly a space and cost-saving measure by manufacturers.
Even so many people recommend buying a spare wheel as replacing the tyre is safer than the other options, plus a repair kit will not help if they tyre has actually burst.
However that may then pose the issue of where to store it if the vehicle is not designed to hold one.
Be sure you definitely do not have a spare as it is not always in the obvious place (such as on the back of the camping car). Places where it may be include attached under the vehicle behind the back axis or (if there is one) in the luggage hold.