[mlir] Migrate away from std::nullopt (NFC)#145523

kazutakahirata merged 3 commits intollvm:mainllvm/llvm-project:mainfrom

kazutakahirata:cleanup_20250624_nullopt_mlir_ArrayRefkazutakahirata/llvm-project:cleanup_20250624_nullopt_mlir_ArrayRefCopy head branch name to clipboard