allow reading rosbag1 connections with no messages

When a rosbag is split into multiple smaller sections, it often happens that rare messages no longer occur in all those sections. Depending on the tool doing the splitting, the connections may or may not remain. This change prevents an assertion from being raised if a connection no longer has any messages in it. This makes rosbags more consistent with the official ROS rosbag tool, which permits/reads these bags without error.