fix 修复 全局处理器不生效问题 根据官方issue改为特殊写法(不理解为什么 https://github.com/apache/fesod/issues/648)

ruoyi-common/ruoyi-common-excel/src/main/java/org/dromara/common/excel/convert/ExcelBigNumberConvert.java

@@ -28,7 +28,7 @@ public class ExcelBigNumberConvert implements Converter<Long> {
 
     @Override
     public CellDataTypeEnum supportExcelTypeKey() {
-        return CellDataTypeEnum.STRING;
+        return null;
     }
 
     @Override