The third parameter is returned only when the first one is not Null so maybe your field is empty like an empty string (not like Null value). However I would recommend you to let your method to return the the link and call it in transformation just like this:
<%# CustomFunctions.ReturnUrlSrc(Eval("ProductsVideoLink").ToString()) %>
And in the ReturnUrlSrc
function you check if the parameter is empty or not and then it returns an empty string or the entire <a>...</a> markup. Is it clear?