[mlir] Migrate away from ArrayRef(std::nullopt) (NFC) (#145140)

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.

GitOrigin-RevId: e6ebf8f99ba27d60026c7473b0cd5e24c855b018