I don't think you can get what you want if you don't have access to the site administration. You will therefore need the collaboration of your site administrator.
I see two ways of doing it. Either modify the question, which is perhaps the best solution but which I did not have time to study, or use a small script, a fairly simple solution.
Just place the script below in Site administration / Appearance / Additional HTML / Within HEAD.
Depending on the speed of your server and network, you may need to adjust, i.e. slightly increase the value "2000" in the script, which gives a delay of 2 seconds (2000 ms) for its execution. Indeed, the page has "popout's" which seem to slow down its loading (and do not seem to be very useful ).
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.6.0/jquery.min.js">
</script>
<script>
$(document).ready(function() {
if ($("body").is("#page-mod-assign-grader")) {
setTimeout(function(){
$("td.score input").each(function(){
if ( $(this).attr("value") == "" ) {
$(this).attr("value","0");
}
});
}, 2000);
}
});
</script>