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 18.6.0 (final) on 2025-12-16 between 09:30 and 10:30 UTC+1
This patch makes evolve compatible across Mercurial core commit d55b71393907.