$foo = null;
$Metakey = "Harriet's Adages";
$Metavalue = "wordpress' database interface is like Sunday Morning: Easy.";

if ($foo == null) {
$wpdb->query( $wpdb->prepare( "
    INSERT INTO $wpdb->postMeta
    ( post_id,Meta_key,Meta_value,field_with_null )
    VALUES ( %d,%s,NULL )",10,$Metakey,$Metavalue ) );
} else {
$wpdb->query( $wpdb->prepare( "
    INSERT INTO $wpdb->postMeta
    ( post_id,%s)",$Metavalue,$foo ) );
}