Gettext builder generates empty `msgid` entries from toctrees without caption

[hwhsu1231]

Describe the bug

context

When a Sphinx project using sphinx-external-toc includes subtrees without a caption in _toc.yml, and the gettext builder is used to generate .pot files.

expectation

No empty msgid entries should appear in the generated .pot files.

bug

The insert_toctrees() function in events.py unconditionally sets rawcaption on every toctree node via toctree.caption or "":

sphinx-external-toc/sphinx_external_toc/events.py

Lines 233 to 235 in ba976db

	# TODO this wasn't in the original code,
	# but alabaster theme intermittently raised `KeyError('rawcaption')`
	subnode["rawcaption"] = toctree.caption or ""

When no caption is present, this assigns an empty string "", which Sphinx's extract_original_messages() then extracts as an empty msgid:

#: path/to/file.md:1
msgid ""
msgstr ""

These empty entries cause msgcat and other gettext tools to report "duplicate message definition" errors.

problem

Sphinx's toctree.extract_original_messages() checks 'rawcaption' in self — it only cares whether the key exists, not whether its value is non-empty.

    def extract_original_messages(self) -> list[str]:
        messages: list[str] = []

        # toctree entries
        messages.extend(self.get('rawentries', []))

        # :caption: option
        if 'rawcaption' in self:
            messages.append(self['rawcaption'])
        return messages

The workaround toctree.caption or "" (originally added for alabaster theme compatibility) forces the key to always exist, causing gettext to extract empty strings.

Reproduce the bug

1. Create a _toc.yml with nested subtrees that lack a caption:

   root: intro
   subtrees:
   - entries:
     - file: doc1
       subtrees:
       - entries:
         - file: subfolder/page

2. Run the gettext builder:

   sphinx-build -b gettext . _build/gettext

3. Inspect the generated .pot files — they will contain empty msgid entries.

List your environment

• sphinx-external-toc==1.1.0

Additional context

This issue was originally reported as sphinx-doc/sphinx#13168