When `title` is empty, argument groups are not recognized by toctree

[mondeja]

With this input:

.. sphinx_argparse_cli::
   :module: my_prog.__main__
   :func: build_parser
   :prog: my-prog

Toctree render as:

---

And with this input:

#######################
my-prog - CLI interface
#######################

.. sphinx_argparse_cli::
   :module: my_prog.__main__
   :func: build_parser
   :prog: my-prog
   :title:

Toctree render as:

But the group is added as an <h2> in the page.

[gaborbernat]

Feel free to add a PR to address this 👍