[Scene] Clear block 1 if empty

I think it would be more relevant to close this thread and concentrate the votes on [Scènes] Pouvoir insérer un bloc de scène (plutôt qu'un ajout en fin) - #3 par StephaneB which deals both with removing the first block and inserting a block between two existing ones.

@mutmut If you remove your vote there and add it to the other, I’ll close this thread…