doc[storage]: update document

storage/README.md

@@ -8,6 +8,8 @@ English | [简体中文](./README_CN.md)
 
 - Take advantage of the reflection nature of the Java dynamic language to parse Excel without any code
 
+- null safe, empty strings resolve to strings of length 0, and empty collections resolve to sets of length 0
+
 - Support Excel to export json, csv files
 
 ### Ⅱ. Automatic mapping

storage/README_CN.md

@@ -7,6 +7,8 @@
 
 - 利用Java动态语言的反射特性，无需任何代码就能解析Excel
 
+- null safe，空的字符串解析为长度为0的字符串，空的集合解析为长度为0的集合
+
 - 支持Excel导出json，csv文件
 
 ### Ⅱ. 自动映射