DefaultSolrIndexer sends an empty add request to a remote Solr, causing a "missing content stream" error

While the logged error has no further consequences, the message can be quite annoying and confusing and it is a true regression introduced by XWIKI-23239 which changed the code to submit a list of documents instead of submitting them individually. I've therefore marked the issue according to our rules for regressions and backported the fix to all affected LTS branches.