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>

src/invoicePdf.tsx

@@ -449,8 +449,11 @@ export function InvoiceDocument({ order, seller }: { order: InvoiceOrder; seller
 
         <View style={styles.footer} fixed>
           <Text>
-            {seller.sellerName} · {seller.sellerStreet}, {seller.sellerZip} {seller.sellerCity} · {seller.sellerEmail} · USt-IdNr.{" "}
-            {seller.vatId}
+            {seller.sellerName} · {seller.sellerStreet}, {seller.sellerZip} {seller.sellerCity} · {seller.sellerEmail}
+            {/* Omitted entirely (not "USt-IdNr. " with a blank value) when
+                unset — a Kleinunternehmer (§19 UStG) often has no USt-IdNr.
+                at all, see seller.ts's own comment. */}
+            {seller.vatId ? ` · USt-IdNr. ${seller.vatId}` : ""}
             {seller.registerCourt && seller.registerNumber ? ` · ${seller.registerCourt} · ${seller.registerNumber}` : ""}
             {seller.managingDirector ? ` · Geschäftsführung: ${seller.managingDirector}` : ""}
           </Text>