persistent-nodemap: avoid writing nodemap for empty revlog
There is currently nothing known to write such empty nodemap right now, but the change we are preparing on default reveal this issue. So I rather fix it on stable.
All legacy (hg-git based) Projects have been automatically migrated to native with the upgrade of 2025-01-23
There is currently nothing known to write such empty nodemap right now, but the change we are preparing on default reveal this issue. So I rather fix it on stable.