Make InvoiceSeller.vatId optional for Kleinunternehmer sellers

Payload's company-settings.vatId is no longer hard-required (a
Kleinunternehmer often has no USt-IdNr. at all) — this package's
InvoiceSeller.vatId follows suit, with every renderer now omitting the
USt-IdNr. line/field entirely instead of printing/serializing an empty
value.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -80,6 +80,8 @@ Unlike `vatExempt`, this package does **not** expect `unitPrice` to be de-grosse
 - Visual PDF (`invoicePdf.tsx`/`correctionInvoicePdf.tsx`): the "enthält X% MwSt." annotation under Gesamt becomes "Gemäß § 19 UStG wird keine Umsatzsteuer berechnet." instead — takes precedence over the `vatExempt` note if (implausibly) both were ever set.
 - E-invoice XML (`buildEInvoiceData.ts`): VAT category `E` + a free-text exemption reason, no VATEX code — see "E-invoicing" above.
 
+`InvoiceSeller.vatId` is now optional (`string | null`, was a plain required `string`) for the same reason — a Kleinunternehmer commonly never registers for an USt-IdNr. at all (no intra-EU trade). Every renderer treats a missing value as "omit the USt-IdNr. line/field" rather than printing an empty one: the visual PDF footer drops the whole `· USt-IdNr. …` segment, and `buildEInvoiceData.ts`'s `sellerParty()` omits `cac:PartyTaxScheme` entirely rather than emitting one with a blank `CompanyID`.
+
 ## CI validation (Mustang)
 
 Every push/PR runs `.gitea/workflows/validate-einvoice.yml` against git.mk360.de's own self-hosted Gitea Actions runner (`vps-runner`, registered on the same VPS as Gitea/Payload — see `/home/marco/dev/docker/docker-compose.yml`'s `act_runner` service):