`Ledger::walkLedger` returned `walkMapParallel`'s result directly when asked for a
parallel walk, which skipped the transaction map walk and the missing-node logging
that follow. `Application::loadOldLedger` is the only parallel caller, so `--load`,
`--ledger`, `--ledgerfile` and `--replay` never checked a ledger's transaction map.
The parallel result is now held in a local and combined with both maps' missing-node
lists at the end. The early return could not simply be deleted: `walkMapParallel`
reports a worker that cannot read the node store through its return value alone, so
dropping the boolean would lose that answer while `missingNodes1` stays empty.
This is an operational change for anyone using `--load` as a recovery step. A ledger
whose transaction map is missing a node is now refused at startup rather than
accepted. Refusing is the intent: the walk exists to decide whether every node of
both maps is available, and until now it answered for only one of them.
Two gtests in `src/tests/libxrpl/ledger/WalkLedger.cpp` build a ledger whose state
map is complete and whose transaction map holds only its root, then check that both
the parallel and the serial walk report it incomplete. A second case checks that a
ledger with two complete maps passes on both paths. The first fails against the
early return.
