Handle bag files with empty topics (#666)

When reading an mcap file, make message count default to 0 if it's not
in the mcap summary (when there are no messages).

Co-authored-by: Calvin Winkowski <cwinkowski@vtti.vt.edu>

Author: telnoratti

python/src/ouster/sdk/bag/bag_packet_source.py

@@ -105,7 +105,7 @@ def bag2_monkey(self, paths, default_typestore=None):
     for id, c in f.channels.items():
         con = Connection(id = id, topic=c.topic, msgtype=c.schema,
                          msgdef=schemas[c.schema][1],
-                         msgcount = counts[id],
+                         msgcount = counts.get(id, 0),
                          ext = ConnectionExtRosbag2(serialization_format='cdr',
                                                     offered_qos_profiles=qos.get(c.schema, '')),
                          owner = self, digest = None)