ArrayRef has a constructor that accepts std::nullopt. This
constructor dates back to the days when we still had llvm::Optional.
Since the use of std::nullopt outside the context of std::optional is
kind of abuse and not intuitive to new comers, I would like to move
away from the constructor and eventually remove it.
This patch takes care of the mlir side of the migration, starting with
straightforward places like "return std::nullopt;" and ternally
expressions involving std::nullopt.
e6ebf8f99b