If image file is empty PHP mysql

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PHP

MySQL

Good day.

When I try to edit an existing image, my code works up until when no image is selected. I put in an IF to echo a message 'Sorry, upload a picture.' but this doesn't run. The whole php is below. What I want to achieve is to run an sql with image when image is selected, and an sql without image when none is selected. please help. Thanks in anticipation.

What I have tried:

$mysqli = new mysqli ("localhost", "root", "", "dbname");

if (isset($_POST['update'])) {
    $id = $_POST['id'];
    $surname = $_POST['surname'];
    $other_names = $_POST['other_names'];
    $dob = $_POST['dob'];
    $marital_status = $_POST['marital_status'];
    $telephone = $_POST['telephone'];
    $unit = $_POST['unit'];
	$img_name = $_FILES['picture']['name'];
	$img_size = $_FILES['picture']['size'];
	$tmp_name = $_FILES['picture']['tmp_name'];
	$error = $_FILES['picture']['error'];

    if ($error === 0) {
	if ($img_size > 500000) {
			echo "<script>
            alert('Sorry, your file is too large, it should be less than 0.5MB or 500kb.');
            window.location.href='../view_members.php';
            </script>";
		}else {
			$img_ex = pathinfo($img_name, PATHINFO_EXTENSION);
			$img_ex_lc = strtolower($img_ex);
			$allowed_exs = array("jpg", "jpeg", "png");
	if (in_array($img_ex_lc, $allowed_exs)) {
				$new_img_name = uniqid("IMG-", true).'.'.$img_ex_lc;
				$img_upload_path = '../images/'.$new_img_name;
				move_uploaded_file($tmp_name, $img_upload_path);
            $query = "UPDATE new_members SET surname='$surname', other_names='$other_names', dob='$dob', marital_status='$marital_status', 
            telephone='$telephone', unit='$unit', picture='$new_img_name' WHERE id = $id";}

        else{echo "<script>
            alert('Sorry, only JPG, JPEG & PNG files are allowed.');
            window.location.href='../view_members.php';</script>";}
    
            if($_FILES['picture']['error'] > 0) {
                echo "<script>
            alert('Sorry, upload a picture.');
            window.location.href='../view_members.php';</script>";
          }
}
    if($mysqli->query($query)){
        header("location:../view_members.php?project_edit=success");
}  
}
}
?>