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>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@einfach-produktiv/invoicing",
-  "version": "0.2.0",
+  "version": "0.2.1",
   "private": true,
   "description": "Shared invoice / correction-invoice (Stornorechnung, Gutschrift) PDF generation and VAT-breakdown math, consumed as a git dependency by both the einfach-produktiv frontend and the payload backend — not published to npm.",
   "type": "module",