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
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