adding missing email address for principal

fixes #8515 


I extracted the code part from #8734 which are unrelated.

@libasys Does this fix the reported stuff from #8515 ?
See https://github.com/owncloud/core/issues/11889, plus Scrutinizer actually complained about this...

https://scrutinizer-ci.com/g/owncloud/core/inspections/d58b7042-776c-46d2-be70-25c2e5fcb4e9/issues/files/lib/private/connector/sabre/principal.php?status=new&orderField=path&order=asc