comm: fix NUL --output-delimiter with --total

comm: fix NUL --output-delimiter with --total

* src/comm.c (compare_files): Handle the single character
--output-delimeter case separately so that NUL is appropriately
handled.
* doc/coreutils.texi (comm invocation): Fix the description
of --output-delimiter to say an empty delimeter is treated
as a NUL separator, rather than being disallowed.
* tests/misc/comm.pl: Add a test case.
Reported at https://bugs.debian.org/1014008