No, that didn't work either!
I also tried:
<?php if(isset($_POST["ProductWeight"]) && floatval($_POST["ProductWeight"]) = "") $_POST["ProductWeight"] = "0.01"; ?>
and
<?php if(isset($_POST["ProductWeight"]) && floatval($_POST["ProductWeight"]) == '') $_POST["ProductWeight"] = 0.01; ?>
and
<?php if(isset($_POST["ProductWeight"]) && floatval($_POST["ProductWeight"]) == NULL) $_POST["ProductWeight"] = 0.01; ?>