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 18.0.0rc1 on 2025-06-12 between 19:00 and 20:00 UTC+2
This patch makes evolve compatible across Mercurial core commit d55b71393907.