[flang] Migrate away from std::nullopt (NFC) (#145928)
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 replaces std::nullopt with {}. There are a couple of
places where std::nullopt is replaced with TypeRange() to accommodate
perfect forwarding.
