At present it is not possible to return NULL from WS method because 
external_api::clean_returnvalue() does not allow it for  single and multiple structure.

This is a problem because in cases such as \core_message_external::get_conversation_between_users() where developers incorrectly throw exception instead of returning NULL indicating that no data was found.

if ($conversation) {

    return $conversation;

} else {

    // We have to throw an exception here because the external functions annoyingly

    // don't accept null to be returned for a single structure.

    throw new \moodle_exception('errorconversationdoesnotexist', 'message');

} 

At the same time we could optionally allow NULLs in WS parameters instead of single and multiple structure.

This should be fully backwards compatible.