fix: 空字符串 remark/specification/unit 触发微信支付 API 校验

- remark 空字符串改为不传
- specification/unit 移除（可选字段，不填则不传）

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

service/src/main/java/com/kym/service/wechat/impl/WechatPayFapiaoService.java

@@ -113,7 +113,7 @@ public class WechatPayFapiaoService {
         fapiaoInfo.put("amount", totalAmount - totalTax);
         fapiaoInfo.put("items", items);
         fapiaoInfo.put("invoice_type", "082");
-        if (invoice.getRemark() != null) {
+        if (invoice.getRemark() != null && !invoice.getRemark().isBlank()) {
             fapiaoInfo.put("remark", invoice.getRemark());
         }
 
@@ -138,8 +138,6 @@ public class WechatPayFapiaoService {
         var item = new HashMap<String, Object>();
         item.put("tax_code", taxCode);
         item.put("goods_name", name);
-        item.put("specification", "");
-        item.put("unit", "");
         item.put("quantity", 100000000);
         item.put("unit_price", (long) totalAmount * 10000L);
         item.put("amount", amount);