[mlir][examples] Use {} instead of std::nullopt (NFC)#146949

kazutakahirata merged 2 commits intollvm:mainllvm/llvm-project:mainfrom

kazutakahirata:cleanup_20250703_mlir_example_fixkazutakahirata/llvm-project:cleanup_20250703_mlir_example_fixCopy head branch name to clipboard