Prevent undefined offset error on deleted categories.

[romainruaud]

This one is a workaround for this issue : magento/magento2#10421

This could lead to several errors, since we rely on this table when reindexing products, and provoke failures when trying to index category names into products.

To reproduce :

• Have the indexers in "Update on Save" mode.
• Delete the category YYY where the product XXX is affected.
• Try to execute an action which triggers a reindex of this product (save it in the BO, or event order it in front-office)
• The action will fail, due to "Undefined index: YYY on module-elasticsuite-catalog/Model/ResourceModel/Product/Indexer/Fulltext/Datasource/CategoryData
  .php on line 93"

This is due to the fact we rely on a table where old rows concerning a deleted category are still present.

Regards,