Incorrect NSStringEncoding value 0x0000 detected.

当一个字符串为null时，它不能被正常地使用，否则会报如下错误

Incorrect NSStringEncoding value 0x0000 detected. Assuming NSASCIIStringEncoding. Will stop this compatiblity mapping behavior in the near future

错误案例：当nstrPublicUrl==null时，如果我们写下如下代码，就会报如上警告。

//错误的案例，nstrPublicUrl==nil时
NSString *nstrUrl = [NSString stringWithFormat:@"%@%@",nstrPublicUrl,ACCOUNT_URL];