CLI: show help anytime a command is called with no args

[brimoor]

#382 addresses this for "meta" commands that are only composed of subcommands. I.e., eta auth will automatically show help.

However, eta build does not show help.

The ask is to find a clean to show help anytime an empty argparse.Namespace object is passed to Command.run. But I haven't found a clean way to do that.

Note that it's possible for some commands to intentionally not have arguments. Like voxel51 status, for example, in the API CLI. So, the above logic should not be applied blindly everywhere.