changelog: fix handling of empty copy entries in changeset
Before this patch, when an empty value was found in the changeset, we
would get a ValueError, which would result in None being returned for
addedfiles/removedfiles and p1copies/p2copies. That made 278dcb24e535
(copies: write empty entries in changeset when also writing to
filelog, 2019-04-23) ineffective at helping the read path not look for
copies in the filelogs.
Differential Revision: https://phab.mercurial-scm.org/D6595