PBS 4.x and fresh PVE 9 / Trixie installs no longer ship
/etc/apt/sources.list (deb822 only). Running 'sed -i ... /etc/apt/sources.list || true'
suppresses the exit code but still prints the noisy stderr message
'sed: can't read /etc/apt/sources.list: No such file or directory'
during the post-install routine.
Wrap the call with an existence check so the sed step is skipped cleanly
when the legacy file is absent.
Fixes#14174
c3a5abfa0b