fix: [세금계산서] NOT NULL 컬럼 null 방어 처리

- supplier/buyer corp_num, corp_name null→빈문자열 보정
- Laravel ConvertEmptyStringsToNull 미들웨어로 인한 DB 에러 방지

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Services/TaxInvoiceService.php

@@ -112,6 +112,12 @@ public function create(array $data): TaxInvoice
         // 합계금액 계산
         $data['total_amount'] = ($data['supply_amount'] ?? 0) + ($data['tax_amount'] ?? 0);
 
+        // NOT NULL 컬럼: Laravel ConvertEmptyStringsToNull 미들웨어가 ''→null 변환하므로 보정
+        $data['supplier_corp_num'] = $data['supplier_corp_num'] ?? '';
+        $data['supplier_corp_name'] = $data['supplier_corp_name'] ?? '';
+        $data['buyer_corp_num'] = $data['buyer_corp_num'] ?? '';
+        $data['buyer_corp_name'] = $data['buyer_corp_name'] ?? '';
+
         $taxInvoice = TaxInvoice::create(array_merge($data, [
             'tenant_id' => $tenantId,
             'status' => TaxInvoice::STATUS_DRAFT,