passive agents should be able to send empty strings

With the current protocol, when a server requests a value for a particular item, passive agent is expected to send a raw value in return. That is, if the result value is "abcdef", the agent will only put "abcdef" into the socket and nothing else (not even protocol headers). If the server does not get anything from the agent (which is also the case with an empty string), it assumes the agent dropped connection.