brpc redis客户端不支持空字符串

**Describe the bug (描述bug)**
brpc::RedisRequest request;
std::string value("");
request.AddCommand("set key %b", value.data(), value.size());

**To Reproduce (复现方法)**
以上

**Expected behavior (期望行为)**
设置key的值为空字符串成功, 因为redis本身支持空字符串

**Versions (各种版本)**
OS:
Compiler:
brpc:
protobuf:

**Additional context/screenshots (更多上下文/截图)**
报错为, 参数个数不对, 应该是拼接命令字符串的时候有问题

Start with the RedisRequest::AddCommand call shown in the reproduction and trace how its arguments are assembled into a Redis command. Done means setting a key with an empty string succeeds without an argument-count error; add a regression test if the relevant test location can be found.

Written by the indexing model from the issue text.