Content module will not update to a NULL value

When updating a cck value to NULL, the existing value in the field is not overwritten.

Example:
When using the migrate module to update an existing node, only fields that have content are migrated. If you are updating a node's field property from a text value "some value" to a NULL value, the text in the destination node is not set to null.

Similar / related issues
http://drupal.org/node/227677 (This issue contains the core patch I am using)
http://drupal.org/node/85929

UPDATE:
An update to the patch is attached. The elseif should be changed to only check for false when drupal_property_exists() is called. Checking for is_set is not necessary because we are checking the $object variable only contains properties that are set in the given node. If a property is being set to NULL, it will return false when drupal_property_exists() is called on it because NULL properties do not exist in $obejct. This code is wrapped in a foreach that loops through each value in $schema['fields']. So, $field will contain every known property on $object, not just the ones that are being set.

Example of an object that is being saved. The object contains 3 fields; x, y & z. z is being set to NULL.

When the object is ready to be save, it only contains values for the fields that have something in them:

object(stdClass)#2725 (22) {
["field_x_value"]=>
string(1) "0"
["field_y_value"]=>
string(1) "0"
["nid"]=>
string(6) "260250"
["vid"]=>
string(6) "260334"
}

When we loop through $schema['fields'] for the given $table, all properties are checked, regardless of their value. If we deliberately put a NULL in any of these fields, it won't be saved using the old code.

vid = 260334
nid = 260250
field_x_value = 0
field_y_value = 0

By adding the else if, the code catches the NULL property and adds it to $values

vid = 260334
nid = 260250
field_x_value = 0
field_y_value = 0
field_z_value =

--------
Another way to reproduce this issue:
- ### is a node id that has a cck field
- "test" is the name of the cck field

$ drush php-eval '
$node = node_load(###);
// Try to write empty space to a cck field.
$node->field_test = array(array("value" => ""));
node_save($node);
echo "\ndone\n";'

Outputs this:
Column 'field_test_value' cannot be null
[warning]
query: INSERT INTO content_field_test (vid, nid,
field_mediarecall_metadata_value) VALUES (###, ###, NULL)
database.mysqli.inc:147

done