relsize: also return a valid value for sizes of 0 and provide test case, fixes #4462

git-svn-id: svn://develop.studip.de/studip/trunk@30901 c5b85792-5220-0410-ad23-ed5b6b8ad999
fix messed up test from [30872], re #4412

git-svn-id: svn://develop.studip.de/studip/trunk@30902 c5b85792-5220-0410-ad23-ed5b6b8ad999