To add to Virgil's answer you can also do some checking in the transformation to see if that field has a value or not. If not, then don't do as it is already setup to do and display some default placeholder, just don't display the HTML element(s). Here's an example
<%# IfEmpty(EvalText("SKUImagePath"), "", "<img src='/Site/media/Items/Thumbnails/" + EvalText("SKUImagePath") + "'/>")%>