Show fallback "No indexes found in this documentation." in `genindex.rst`

[alexander-nitsche]

Insert a placeholder text if no index has been added yet to a document. Something like

(1) "No indexes added yet." or (2) "No indexes available." or (3) "No indexes found in this documentation."

so that the HTML looks like:

<div class="document" itemscope="itemscope" itemtype="http://schema.org/Article" role="main">
<div itemprop="articleBody">
<!-- body -->
<h1 id="index">Index</h1>
<div class="genindex-jumpbox">
No indexes found in this documentation.
</div>
<!-- /body -->
</div>
</div>

and the rendered content, instead of

---

---

like this:

---

---

	URL
Live example of empty index	https://docs.typo3.org/m/typo3/guide-frontendlocalization/main/en-us/genindex.html
Live example of filled index	https://docs.typo3.org/m/typo3/reference-coreapi/main/en-us/genindex.html

[flicstar]

Index refers to the collection, so I don't think we want to make it plural here. I would go for something more like:

• No index terms found in this documentation.
• No terms have been indexed yet in this documentation.
• There are no terms indexed in this documentation.

[marble]

Solved with DRC v3.0.dev26. When rendering for the server docs.typo3.org this is what you get: