rust-changelog: remove special parsing of empty changelog data for null rev For the null revision, `Revlog::get_rev_data()` will return an empty string (of bytes). We currently handle that case in `ChangelogRevisionData::manifest_node()`. However, it's going to be ugly to have special handling for the null revision for each future method on `ChangelogRevisionData`. This patch therefore restructures the code so we instead initialize the struct with valid data for the null revision. Differential Revision: https://phab.mercurial-scm.org/D12438