Report Builder empty values (Custom Expressions, group_concat) 7.01 iFix13

I'm trying to check if an requirement is linked or not. For that I'm using a custom expression.

IF($Req$!="","Linked","NotLinked"). Linked is diplayed but NotLinked not.

I have a similar issue with using group concat. I'm using it to fetch additional data related to requirement without creating additional lines for the same requirement. I'm grouping for instance all test results for the requirements which are satisfying the requirement in one line. so that I have one entry per requirement but still the information if one of the test cases of the child requirments might have failed or not and which test case id is affected.

However if I there is a child requirement without a test case the test ids remain emtpy even if there is another relevant requirment which has a test case