auto tree = ryml::parse_in_arena("a: ''");
std::string emptyString;
tree["a"] >> emptyString; //throws

According to the standard, if I'm understanding it correctly, empty nodes should be interpreted as null, unless you're deserializing into a string, in which case they should be interpreted as 0-length (but not null) strings. This means that when deserializing a node into a ryml::csubstr or std::string, a 0-length scalar is a valid 0-length string value.