You can set a callback function for the onSubmit event of the form and check the contents of each field. If it contains nothing you can then fill it with the string "empty":
<form name="my_form" action="validate.php" onsubmit="check()">
<input type="text" name="text1" />
<input type="submit" value="submit" />
</form>
and in your js:
function check() {
if(document.forms["my_form"]["text1"].value == "")
document.forms["my_form"]["text1"].value = "empty";
}