--null does nothing when used with --cat or --pipe

[Ian Turner]

parallel --null does the right thing normally, but when used with --cat or --pipe, seems to have no effect. Some examples below.

Without --cat or --pipe:
$ echo -e 'a\0b\0c' | parallel --will-cite --null echo | xxd
00000000: 610a 620a 630a 0a a.b.c..
We see the null bytes transformed to newlines, indicating three invocations of 'echo' with 'a", "b", and "c\n", respectively.

With --cat or --pipe:
$ echo -e 'a\0b\0c' | parallel --will-cite --null --cat xxd
00000000: 6100 6200 630a a.b.c.
$ echo -e 'a\0b\0c' | parallel --will-cite --null --pipe xxd -
00000000: 6100 6200 630a a.b.c.
We see the output is the same as the input, indicating the input was passed unchanged to xxd.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: parallel 20161222-1
ProcVersionSignature: Ubuntu 4.15.0-23.25-generic 4.15.18
Uname: Linux 4.15.0-23-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
CurrentDesktop: KDE
Date: Mon Jul 2 16:41:41 2018
InstallationDate: Installed on 2018-03-23 (101 days ago)
InstallationMedia: Kubuntu 18.04 LTS "Bionic Beaver" - Alpha amd64 (20180306.1)
PackageArchitecture: all
SourcePackage: parallel
UpgradeStatus: No upgrade log present (probably fresh install)