RGB, Depth, and Point Cloud topics should not automatically subscribe

[dcolli23]

The RGB, depth, and point cloud topics should not automatically subscribe to the topic if no topic is given. This leads to having to specify rgb, depth, and point cloud topics every time we run cdcpd even if we only want to use point clouds.

I imagine defaulting the topic names to an empty string and checking if they're empty before subscribing would be a sufficient implementation.