POST empty fields result in query error - php

Question

I have a form with some fields
<form action="add.php" method="post">
/*Some fields*/
</form>
Also I created the following function to quote and escape form submitted values
<?php
// Quote and escape form submitted values
function db_quote($value) {
$connection = db_connect();//Connection with database "NO ISSUE HERE"
return "'" . mysqli_real_escape_string($connection,$value) . "'";//Maybe the issue here
}
?>
Then I pass values to the function as following
$inventoryId = db_quote($_POST['inventoryId']);
$sn = db_quote($_POST['sn']);
$model = db_quote($_POST['model']);
//etc...
Every thing work fine if I fill all the fields, but if there is at least one empty filed i.e. no value entered by the user I get the following error
Catchable fatal error: Object of class mysqli could not be converted to string in etc...
Here is the query I am trying to run
<?php
$sql = "INSERT INTO inventory (id,manufacturer_id,supplier_id,servicer_id,operator_id,sn,model,inventory_name,inventory_type,description,power,purchase_order,purchase_cost,arrival_date,installation_date,warranty_date,incident_history,conditions,m_next_date,m_start_date,m_deadline,lifetime,inspection_frequency,location,purchased_from)
VALUES ($inventoryId,$manufacturer,$supplier,$servicesId,$operatorId,$sn,$model,$inventoryName,$inventorType,$description,$power,$purchaseOrder,$purchaseCost,$arrivalDate,$installationDate,$warranty,$incident,$conditions,$nextDate,$startDate,$deadline,$lifetime,$inspection,$location,$purchasedFrom);";
if ($connection->query($sql) === TRUE) {
echo "<p>New inventory ".$inventoryId." created successfully</p>";
} else {
echo "Error: " . $sql . "<br>" . $connection;
}
$connection->close();
?>
Update: The issue only with Auto Increment columns

Answer 1

<?php
// Quote and escape form submitted values
function db_quote($value) {
$connection = db_connect();//Connection with database "NO ISSUE HERE"
return "'" . mysqli_real_escape_string($connection,$value) . "'";//Maybe the issue here
}
?>
Convert it to
<?php
// Quote and escape form submitted values
function db_quote($value) {
if(!$value) return '';
$connection = db_connect();//Connection with database "NO ISSUE HERE"
return "'" . mysqli_real_escape_string($connection,$value) . "'";//Maybe the issue here
}
?>