bufbuild/protovalidate 如何判断 string 是否为空跟未设置

[ohohohh]

当 string username = "跟未设置 username ，如何让检验返回自定义错误？

string username = 2 [
(buf.validate.field).cel = {
id: "username.not_empty",
message: "username not Empty",
expression: "this.size() > 1"
}
];

[qzzz]

optional string bar = 1;