PDF export requires home directory for user running xwiki server

If you try to export a page as pdf you will get an exception if the user running xwiki server has no home directory on linux.

see also: https://forum.xwiki.org/t/pdf-export-issue/1354

As it is quite common to create restricted service users in linux without a home directory the fop caches should either be moved or there should be a configuration setting that allows moving the caches out of the (non-existent) homedirectory.