And this is the most important point, the contract you would have with Qatar could be to take you to an EU airport, and when you check in they have to verify that you are admissable to their contracted destination. They might take into account onward travel and the opportunity of an airside transit, but they don't have to, and their T&C's will put the onus on the passenger to ensure they are correctly documented (immigration, health) for the contracted destination.