Avoid printing empty tables and panels

[pederhan]

In order to avoid printing empty panels and tables, we should have a utility function that checks if a renderable is empty, and if so print a message like "No data..." or something similar.

In the case of tables, we could simply check the Table.row_count attribute to check if the table has any rows. For Panels, we would have to check every renderable inside the panel, probably recursively in case the panel contains another panel or group.

Something like:

def is_empty(renderable: RenderableType) -> bool:
    """Check if a renderable is empty."""
    if isinstance(renderable, Table):
        return renderable.row_count <= 0
    elif isinstance(renderable, Panel):
        return is_empty(renderable.renderable)
    elif isinstance(renderable, Group):
        for child in renderable.children:
            if not is_empty(child):
                return False
    else:
        return False
    return True

Although it would probably turn out to be quite a bit more complex than this in practice.