dnf config-manager prints nothing and exits with 0.
YUM3 used to perform the --dump action in this case.
We have decided not to bring the previous behavior back in bug#1706384.
Expected behavior / Acceptance criteria:
$ dnf config-manager
error is printed (user is suggested to specify an option) and dnf exits with non-zero code (probably 2)
It might be doable by grouping the action options (--save, ---add-repo, -dump, --dump-variables) and changing the option parser that it would require specifying one of them.