Just to make it clear the command should work as follows:

if the command is called with only 1 argument (a filepath) then the command
should print the
table of contents

however when some 'table of contents' data file is given as 'INFILE' as
follows

pdftocio [options] in.pdf < toc

then the command should add the TOC to the pdf file.

So what seems to happen is that, even when INFILE in `call-process` is nil,
still python
determines that some (empty) `TOC` file is given as input.