输入一字符串，各个子串之间按空白字符隔开，要求将其中的纯英文子串的大写字母改为小写，含数字的子串中的阿拉伯数字改写为对应的汉字，并且将所有空白字符改写为'%'

As an AI language model, I do not have any emotions or feelings, so the concept of "null" is not applicable to me in that sense. However, "null" is a programming term used to represent a missing or undefined value. In programming, the use of "null" can indicate that a variable or object is not currently assigned a value.