bug#57854: some extra info from inspecting the `pdftocio` package

[dalanicolai <dalanicolai@gmail.com>]

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When looking into the `pdftocio` package I find the following piece of code

```
    try:
        with open_pdf(path_in) as doc:
            if toc_file.isatty() or print_toc:
                # no input from user, switch to output mode and extract the
toc
                # of pdf
                toc = read_toc(doc)
                if len(toc) == 0:
                    print("error: no table of contents found",
file=sys.stderr)
                    sys.exit(1)

                if readable:
                    print(pprint_toc(toc))
                else:
                    print(dump_toc(toc), end="")
                sys.exit(0)

            # an input is given, so switch to input mode
            toc = parse_toc(toc_file)
            write_toc(doc, toc)
```

As from using `shell-command-to-string` we find that `len(toc)` must be 0
(it returns an empty string), so then it is probably the case that python
determines that some input has been given.

So maybe, even though in the `call-process` the INFILE command is set to
`nil`, still python might
determine that some kind of input is given.

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