Replacing an empty string with NULL

I have an form where someone can choose to upload a photo. If the person does not upload a photo, an emtpy string is inserted in the DB. How can I have it insert a NULL value instead of an empty string when they choose NOT to upload a photo but complete the form anyway?

Here is my query:
$query = "INSERT INTO routes(user_id , date_climbed , route , pitchs , rating, area, outcrop, type, style, private_comments, public_comments, stars, route_photo) VALUES('NULL', '$_POST[date_climbed]', '$_POST[route]', '$_POST[pitchs]', '$_POST[rating]', '$_POST[area]', '$_POST[outcrop]', '$_POST[type]', '$_POST[style]', '$_POST[private_comments]', '$_POST[public_comments]', '$_POST[stars]', '$_POST[$route_photo]')";

Much Thanks -- CH

Use an if statement to test if there was a photo uploaded then build the sql query with a NULL instead of the $_POST value if there wasn't one.

-KM-