render empty arrays as "[]", refs #3484

git-svn-id: svn://develop.studip.de/studip/trunk@29021 c5b85792-5220-0410-ad23-ed5b6b8ad999
fix contact group functions, refs #3484

git-svn-id: svn://develop.studip.de/studip/trunk@29022 c5b85792-5220-0410-ad23-ed5b6b8ad999