usually NULL MySQL values will just show up blank, like you want, but if they aren't you could do something like this:
assuming you just got your results from the database in the form of an array and put them in a variable, lets say called $row, and lets say the MySQL feild is called "title":
if ($row['title'] == NULL || $row['title'] == "NULL") {
$row['title'] = "";
}
That should get your started.