fix(billing): support customers without subjects (#3734)

Author: hekike

openmeter/billing/errors.go

@@ -50,7 +50,7 @@ var (
 	ErrInvoiceLineNoPeriodChangeForSplitLine                     = NewValidationError("invoice_line_no_period_change_for_split_line", "invoice line period cannot be changed for split lines")
 	ErrInvoiceLineProgressiveBillingUsageDiscountUpdateForbidden = NewValidationError("invoice_line_progressive_billing_usage_discount_update_forbidden", "usage discount cannot be updated on a partially invoiced line")
 	ErrInvoiceCreateNoLines                                      = NewValidationError("invoice_create_no_lines", "the new invoice would have no lines")
-	ErrInvoiceCreateUBPLineCustomerHasNoSubjects                 = NewValidationError("invoice_create_ubp_line_customer_has_no_subjects", "creating an usage based line: customer has no subjects")
+	ErrInvoiceCreateUBPLineCustomerUsageAttributionInvalid       = NewValidationError("invoice_create_ubp_line_customer_has_no_subjects", "creating an usage based line: customer usage attribution is invalid")
 	ErrInvoiceCreateUBPLinePeriodIsEmpty                         = NewValidationError("invoice_create_ubp_line_period_is_empty", "creating an usage based line: truncated period is empty")
 	ErrInvoiceLineCurrencyMismatch                               = NewValidationError("invoice_line_currency_mismatch", "invoice line currency mismatch")
 

openmeter/billing/service/lineservice/usagebasedline.go

@@ -59,9 +59,9 @@ func (l usageBasedLine) Validate(ctx context.Context, targetInvoice *billing.Inv
 		return err
 	}
 
-	if targetInvoice.Customer.UsageAttribution == nil || len(targetInvoice.Customer.UsageAttribution.SubjectKeys) == 0 {
+	if err := targetInvoice.Customer.Validate(); err != nil {
 		return billing.ValidationError{
-			Err: billing.ErrInvoiceCreateUBPLineCustomerHasNoSubjects,
+			Err: billing.ErrInvoiceCreateUBPLineCustomerUsageAttributionInvalid,
 		}
 	}