changelog: make copies related function return None or a valid value
With the previous code, existing but empty value were not "decoded", leading to
the method returning one of `None`, some valid value (`list` or `dict`) or
`b''`.
On a general basis, not explicitly checking for None is a source of bugs.
Having a clean return types will help the side-data copies code in future
changesets.
Differential Revision: https://phab.mercurial-scm.org/D7037
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.