update 全局替换为 Convert.toStr，优化 null 字符串处理

ruoyi-common/ruoyi-common-excel/src/main/java/org/dromara/common/excel/core/DropDownOptions.java

@@ -1,5 +1,6 @@
 package org.dromara.common.excel.core;
 
+import cn.hutool.core.convert.Convert;
 import cn.hutool.core.util.StrUtil;
 import lombok.AllArgsConstructor;
 import lombok.Data;
@@ -65,7 +66,7 @@ public class DropDownOptions {
         StringBuilder stringBuffer = new StringBuilder();
         String regex = "^[\\S\\d\\u4e00-\\u9fa5]+$";
         for (int i = 0; i < vars.length; i++) {
-            String var = StrUtil.trimToEmpty(String.valueOf(vars[i]));
+            String var = StrUtil.trimToEmpty(Convert.toStr(vars[i]));
             if (!var.matches(regex)) {
                 throw new ServiceException("选项数据不符合规则，仅允许使用中英文字符以及数字");
             }