Fix issue where Sub-commands: would show where no definition items exist...

https://github.com/tony/tmuxp/blob/a07a8fb638979d/tmuxp/cli.py#L688  Here is my parser. It's a little complicated.  I figured you could look at it, in case it was me using ``argparse`` incorrectly.

When I use the config like:

```rest

.. argparse::
   :module: tmuxp.cli
   :func: get_parser
   :prog: tmuxp
   :path: load
```

Or any usage *with* ``:path:`` as an option.

*without* ``:path:``, this works as expected. My subcommands for ``tmuxp load``, ``tmuxp import``, etc show.