small fix in the samples

if the prog is used like : prog csv_file ( so, argc==2 only ) , then :

    string output_folder;
    if (argc == 3) {
        output_folder = string(argv[2]);
    }

leaves output_folder uninitialized, so, later:

   imwrite(format("%s/eigenface_%d.png", output_folder.c_str(), i), norm_0_255(cgrayscale));

will either crash, or write to root