优化导出append方法

src/Tools/Exporter.php

@@ -237,7 +237,7 @@ class Exporter
                 if (isset($data[$v])) {
                     $result[] = $data[$v];
                 } else {
-                    new \Exception("key为{$v}的数据不存在");
+                    $result[] = '';
                 }
             } else {
                 $separate = explode("|", $v);
@@ -246,7 +246,7 @@ class Exporter
                 $t = count($list) - 1;
                 $b = $data;
                 foreach ($list as $lk => $lv) {
-                    if (isset($b[$lv])) {
+                    if (isset($b[$lv]) && $b[$lv]) {
                         $b = $b[$lv];
                     } else {
                         $b = "";