IndexError when generating toctree

[nicki-krizek]

Commit 9f2c660 introduced some issue/behavior change that results in failed doc builds. I don't understand breathe/doxygen sufficiently to be able to provide a minimal reproducer, but I can reproduce this issue 100 % of time when building Knot Resolver docs.

Here's the traceback I get:

Traceback (most recent call last):
  File "/usr/lib/python3.9/site-packages/sphinx/events.py", line 111, in emit
    results.append(listener.handler(self.app, *args))
  File "/usr/lib/python3.9/site-packages/sphinx/environment/collectors/toctree.py", line 129, in process_doc
    toc = build_toc(doctree)
  File "/usr/lib/python3.9/site-packages/sphinx/environment/collectors/toctree.py", line 109, in build_toc
    sub_item = build_toc(sectionnode, depth + 1)
  File "/usr/lib/python3.9/site-packages/sphinx/environment/collectors/toctree.py", line 109, in build_toc
    sub_item = build_toc(sectionnode, depth + 1)
  File "/usr/lib/python3.9/site-packages/sphinx/environment/collectors/toctree.py", line 109, in build_toc
    sub_item = build_toc(sectionnode, depth + 1)
  File "/usr/lib/python3.9/site-packages/sphinx/environment/collectors/toctree.py", line 94, in build_toc
    nodetext = visitor.get_entry_text()
  File "/usr/lib/python3.9/site-packages/docutils/transforms/parts.py", line 162, in get_entry_text
    return self.get_tree_copy().children
  File "/usr/lib/python3.9/site-packages/docutils/nodes.py", line 2103, in get_tree_copy
    return self.parent[0]
IndexError: list index out of range

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/lib/python3.9/site-packages/sphinx/cmd/build.py", line 280, in build_main
    app.build(args.force_all, filenames)
  File "/usr/lib/python3.9/site-packages/sphinx/application.py", line 352, in build
    self.builder.build_update()
  File "/usr/lib/python3.9/site-packages/sphinx/builders/__init__.py", line 296, in build_update
    self.build(to_build,
  File "/usr/lib/python3.9/site-packages/sphinx/builders/__init__.py", line 310, in build
    updated_docnames = set(self.read())
  File "/usr/lib/python3.9/site-packages/sphinx/builders/__init__.py", line 417, in read
    self._read_serial(docnames)
  File "/usr/lib/python3.9/site-packages/sphinx/builders/__init__.py", line 438, in _read_serial
    self.read_doc(docname)
  File "/usr/lib/python3.9/site-packages/sphinx/builders/__init__.py", line 478, in read_doc
    doctree = read_doc(self.app, self.env, self.env.doc2path(docname))
  File "/usr/lib/python3.9/site-packages/sphinx/io.py", line 221, in read_doc
    pub.publish()
  File "/usr/lib/python3.9/site-packages/docutils/core.py", line 219, in publish
    self.apply_transforms()
  File "/usr/lib/python3.9/site-packages/docutils/core.py", line 200, in apply_transforms
    self.document.transformer.apply_transforms()
  File "/usr/lib/python3.9/site-packages/sphinx/transforms/__init__.py", line 86, in apply_transforms
    super().apply_transforms()
  File "/usr/lib/python3.9/site-packages/docutils/transforms/__init__.py", line 171, in apply_transforms
    transform.apply(**kwargs)
  File "/usr/lib/python3.9/site-packages/sphinx/transforms/__init__.py", line 382, in apply
    self.app.emit('doctree-read', self.document)
  File "/usr/lib/python3.9/site-packages/sphinx/application.py", line 462, in emit
    return self.events.emit(event, *args, allowed_exceptions=allowed_exceptions)
  File "/usr/lib/python3.9/site-packages/sphinx/events.py", line 119, in emit
    raise ExtensionError(__("Handler %r for event %r threw an exception") %
sphinx.errors.ExtensionError: Handler <bound method TocTreeCollector.process_doc of <sphinx.environment.collectors.toctree.TocTreeCollector object at 0x7f557440d0a0>> for event 'doctree-read' threw an exception (exception: list index out of range)

Extension error (sphinx.environment.collectors.toctree):
Handler <bound method TocTreeCollector.process_doc of <sphinx.environment.collectors.toctree.TocTreeCollector object at 0x7f557440d0a0>> for event 'doctree-read' threw an exception (exception: list index out of range)