Fix typo from stellar#237

I didn't adapt the logic right.

This was hard to debug because, in integration tests, it caused the
daemon to run `d.logger.WithError(err).Fatal("could not build
migrations")`, which in turn runs `os.Exit` which ends the test right
away (without flushing logs etc).

I need to think how to improve this. One option is to
override the `ExitFunc` in logrus with an assertion.

cmd/soroban-rpc/internal/db/migration.go

@@ -131,12 +131,12 @@ func newGuardedDataMigration(
 	}
 	metaKey := "Migration" + uniqueMigrationName + "Done"
 	previouslyMigrated, err := getMetaBool(ctx, migrationDB, metaKey)
-	if err != nil && !errors.Is(err, ErrEmptyDB) {
+	if !errors.Is(err, ErrEmptyDB) {
 		err = errors.Join(err, migrationDB.Rollback())
 		return nil, err
 	}
 	latestLedger, err := NewLedgerEntryReader(db).GetLatestLedgerSequence(ctx)
-	if errors.Is(err, ErrEmptyDB) {
+	if !errors.Is(err, ErrEmptyDB) {
 		err = errors.Join(err, migrationDB.Rollback())
 		return nil, fmt.Errorf("failed to get latest ledger sequence: %w", err)
 	}