Duration shows up with nodes that don't use duration field

In order when views module retrieves data using LEFT JOINS from nodes that don't use the duration module all values are set to NULL.
Regardless whether duration_create($duration_iso_string = NULL) is called using a valid parameter or not it normally creates a valid duration object.

In duration_field(..) all items are treated the same way and the object duration_create(..) returns will always be valid:

  function __construct($duration_iso_string = NULL) {
    // Easy way: no string given, let's just initialize a zero duration.
    if (!isset($duration_iso_string)) {
      $this->is_negative = FALSE;
      $this->duration = array();
      $this->is_valid = TRUE;
      return;
    }

In my opinion there shouldn't shown up a value for nodes that don't use this field. That's why I set the category to "bug report".

I thought about checking the $item['approx_seconds'] value for existence before calling duration_create(..).

Patch attached,

greetings,
sevi