fix: preserve billing response shape

backend/internal/billing/service.go

@@ -53,6 +53,9 @@ func (s *Service) Overview(ctx context.Context, organizationID, accountID string
 	if err != nil {
 		return Overview{}, err
 	}
+	if ledger == nil {
+		ledger = []LedgerEntry{}
+	}
 	personal, err := s.store.BillingLedger(ctx, organizationID, accountID, 500)
 	if err != nil {
 		return Overview{}, err
@@ -118,6 +121,18 @@ func (s *Service) AdminOverview(ctx context.Context) (AdminOverview, error) {
 	if err != nil {
 		return AdminOverview{}, err
 	}
+	if organizations == nil {
+		organizations = []Organization{}
+	}
+	if members == nil {
+		members = []Member{}
+	}
+	if ledger == nil {
+		ledger = []LedgerEntry{}
+	}
+	if rules == nil {
+		rules = []PriceRule{}
+	}
 	byOrganization := map[string]Wallet{}
 	for _, wallet := range wallets {
 		byOrganization[wallet.OrganizationID] = wallet