compat: make compatible with upstream change to put nullid on repo
This patch makes evolve compatible across Mercurial core commit d55b71393907.
This instance will be upgraded to Heptapod 18.5.1 on 2025-11-13 between 17:00 and 18:00 UTC+2
This patch makes evolve compatible across Mercurial core commit d55b71393907.