changelog: define changelogrevision.p[12]copies for null revision

Authored by martinvonz.

Description

changelog: define changelogrevision.p[12]copies for null revision

Looks like I missed these in 5382d8f8530b (changelog: parse copy
metadata if available in extras, 2017-12-27). `hg debugp[12]copies -r
null` fails before this patch.

Differential Revision: https://phab.mercurial-scm.org/D6376