mcap reader: Use extracted topic

[benemer]

If no topic is provided, the self.check_topic is supposed to extract it, for example, if there is only one message of type PointCloud2. However, we did not pass the inferred topic. This is the fix.

[nachovizzo]

How come 🤔 I thought it was working but maybe I'm confused with lidar visualizer

[benemer]

Same thing there actually: https://github.com/PRBonn/lidar-visualizer/blob/3884578c3943c28b4bfba4ad8ca96a7fb3eca27a/src/lidar_visualizer/datasets/mcap.py#L51C1-L52C1

How to reproduce the error I faced: I had a mcap file with just one PointCloud2 message and did not specify the topic. Doing this, there was no warning that the topic should be passed (because it could extract it), but the resulting topic passed to the reader was empty.

So probably this did not fail before because we always passed the topic.