[llvm] [LLVM] Use {} instead of std::nullopt to initialize empty ArrayRef (PR #109133)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

> @kazutakahirata what do you think?

Great idea!  Thank you for cleaning this up!  The use of `std::nullopt` for `ArrayRef` is a historical artifact.  We used to have `llvm::None` for `llvm::Optional`, and we used `llvm::None` for empty `ArrayRef`, too.  It's a bad idea to use `std::nullopt` for anything other than empty `std::optional`.

https://github.com/llvm/llvm-project/pull/109133