สอบถามการส่งค่าว่างจากฟอร์ม เกี่ยวกับการอับโหลดรูปครับ

$sql = "update table set field1='" . $field1 . "'";
if(!empty($_FILES['filefield1']['name'])){
$sql .= ", picture1 = '" . $_FILES['filefield1']['name'] . "'";
}
if(!empty($_FILES['filefield2']['name'])){
$sql .= ", picture2 = '" . $_FILES['filefield2']['name'] . "'";
}
if(!empty($_FILES['filefield3']['name'])){
$sql .= ", picture3 = '" . $_FILES['filefield3']['name'] . "'";
}
if(!empty($_FILES['filefield4']['name'])){
$sql .= ", picture4 = '" . $_FILES['filefield4']['name'] . "'";
}
$sql .= " where id = " . $_POST['id'];