Translated pages sometimes aren't indexed during extension installation

Steps to reproduce:

• Install an extension that contains translations
• Check if all translations have been indexed in the Solr search index

Simpler: Execute the SolrIndexerIT integration test

Expected result:

All translations have been index/the test passes.

Actual result:

Sometimes, in particular on MySQL, the test fails.

Locally, the failures with MySQL were very reproducible on the stable-17.8.x branch before 17.8.0RC1. Adding debug logs revealed that the translations are indexed at a point where the default document doesn't exist yet. In this condition, indexing them fails. As last step, the default document is created and indexed, but then the translations aren't indexed anymore.

This is fixed on the next restart of XWiki. Even before that, there shouldn't be any consequences as we normally don't rely on translations being indexed in Solr.