Form sends blank/null serialized data - php

Question

I've got an HTML form that uses jQuery serialize method and ajax to post results using PHP into a sqlite database. I've set the jQuery to run every minute to act as an autosave feature so users can return later to finish what they started. Because of the autosave, I can't force all inputs be required.
It works 99% of the time. However, in a few cases, the application sometimes posts blank data to the database even though there is data in the input fields. I've checked to make sure all the HTML input fields have name attributes. Any ideas why it works most of the time but in a few random cases, it doesn't?
I wish I had more to report why it doesn't work all the time. I haven't been able to replicate the bug myself, I'm just going on reports of users. But I know the form is posting blanks into the database because when I go into the database, it says " " instead of "null". So I know the database is receiving something, but it isn't receiving the data the user typed.
HTML
<form action="" method="post" id="evaluationForm">
<!-- this input is disabled since this user can't edit it -->
<label for="FirstName">First Name</label>
<input type="text" name="FirstName" value="<?php echo htmlspecialchars($data['FirstName']);?>" disabled >
<label for="WorkHabitsCommentsSuper">Comments </label><br>
<textarea name="WorkHabitsCommentsSuper" placeholder="Comments are optional">
<?php echo htmlspecialchars($data['WorkHabitsCommentsSuper']);?>
</textarea>
<label for="GoalsSuper">More Comments</label>
<textarea name="GoalsSuper" required>
<?php echo htmlspecialchars($data['GoalsSuper']);?>
</textarea>
<!-- and a whole bunch more fields but you get the idea -->
</form>
JavaScript
function saveEval() {
var datastring = $("form").serialize();
$.ajax({
type: "POST",
url: "autosave-s.php",
data: datastring,
success: function(text) {
if (text == "success") {
alert('It saved. Hooray!');
} else {
alert('Oh no. Something went wrong.');
}
}
});
}
window.onload = function() {
setInterval("saveEval()", 60000)
}
PHP
$db = new PDO('sqlite:evals.sqlite');
$sql = "UPDATE table SET
WorkHabitsCommentsSuper = :WorkHabitsCommentsSuper,
GoalsSuper = :GoalsSuper";
$query = $db->prepare($sql);
$query->execute(array(
':WorkHabitsCommentsSuper' => $_POST['WorkHabitsCommentsSuper'],
':GoalsSuper' => $_POST['GoalsSuper']
));
echo "success";

Answer 1

if(!in_array("",$_POST)){
$db = new PDO('sqlite:evals.sqlite');
$sql = "UPDATE table SET
WorkHabitsCommentsSuper = :WorkHabitsCommentsSuper,
GoalsSuper = :GoalsSuper";
$query = $db->prepare($sql);
$query->execute(array(
':WorkHabitsCommentsSuper' => $_POST['WorkHabitsCommentsSuper'],
':GoalsSuper' => $_POST['GoalsSuper']
));
echo "success";
}else{
echo "Empty";
}
This will check if the posting data is not empty if empty any of the field it will not update and will send 'Empty' response so alert on empty data posting.