you can add IS NOT NULL to the clause:

WHERE substring(hd1.workitem_comment, 115, 1) IN ('1','2','3','4','5') AND hd1.workitem_comment IS NOT NULL

or, if that doesn't work the way you want, if you can post some samples, i can give it another shot...

edit: just thought of something else, too:

WHERE substring(ISNULL(hd1.workitem_comment, ''), 115, 1) IN ('1','2','3','4','5')