SCORU/Node: fix legacy run command (empty operators)
Context
Fixes #5475 (closed).
Depends on !8544 (merged).
This MR (re)allows to run a rollup node with the legacy run command.
Manually testing the MR
dune exec tezt/tests/main.exe -- -f sc_rollup.ml -t 'Alpha: arith - maintenance of inbox in the rollup node (basic)'