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