Empty Numeric Fields cause SQL errors

I have tried to reproduce this, but I didn't got this error. I tried it with both a decimal and a float field and the following XML file:

<?xml version="1.0" ?>
<root>
  <item>
    <title>Item 1</title>
    <float>3.42</float>
  </item>
  <item>
    <title>Item 2</title>
    <float></float>
  </item>
</root>

Can you provide the following:

• An export of your feeds importer.
• The source file you were trying to import.
• An export of your content type + field bases and field instances and put inside a feature module. (The feeds importer may be put in the feature as well.)

The fourth parameter of the mapping target callback $value is an array since Feeds 7.x-2.0-beta1. This was done to handle multivalued fields in consistent way. Therefore, your node object will for example become $node->food_n_factor = array(0 => ''). See also the change record.

Since your targets are single-valued, you could change your code to the following, for example:

function food_import_n_factor ($source, $entity, $target, array $values, $mapping) {
  $entity->$target = reset($values);
}

And instead of the hook hook_feeds_processor_targets_alter() you can now also use the hook hook_feeds_processor_targets() to specify new targets.

The documentation for the hook hook_feeds_processor_targets is in feeds.api.php. It has one parameter less.

/**
 * Adds mapping targets for processors.
 *
 * This hook allows additional target options to be added to the processors
 * mapping form.
 *
 * If the key in $targets[] does not correspond to the actual key on the node
 * object ($node->key), real_target MUST be specified. See mappers/link.inc
 *
 * For an example implementation, see mappers/text.inc
 *
 * @param string $entity_type
 *   The entity type of the target, for instance a 'node' entity.
 * @param string $bundle
 *   The entity bundle to return targets for.
 *
 * @return array
 *   Array containing the targets to be offered to the user. This function must
 *   return an array, even an empty one.
 */
function hook_feeds_processor_targets($entity_type, $bundle) {
  $targets = array();

  if ($entity_type == 'node') {
    $targets['my_node_field'] = array(
      'name' => t('My custom node field'),
      'description' => t('Description of what my custom node field does.'),
      'callback' => 'my_module_set_target',

      // Specify both summary_callback and form_callback to add a per mapping
      // configuration form.
      'summary_callbacks' => array('my_module_summary_callback'),
      'form_callbacks' => array('my_module_form_callback'),
    );
    $targets['my_node_field2'] = array(
      'name' => t('My Second custom node field'),
      'description' => t('Description of what my second custom node field does.'),
      'callback' => 'my_module_set_target2',
      'real_target' => 'my_node_field_two', // Specify real target field on node.
    );
    $targets['my_node_field3'] = array(
      'name' => t('My third custom node field'),
      'description' => t('Description of what my third custom node field does.'),
      'callback' => 'my_module_set_target3',

      // Set optional_unique to TRUE and specify unique_callbacks to allow the
      // target to be unique. Existing entities can be updated based on unique
      // targets.
      'optional_unique' => TRUE,
      'unique_callbacks' => array('my_module_mapper_unique'),

      // Preprocess callbacks are called before the actual callback allowing you
      // to prepare values on the entity or mapping array.
      'preprocess_callbacks' => array('my_module_preprocess_callback'),
    );
    $targets['deprecated_target'] = array(
      'name' => t('A target that cannot be chosen in the UI.'),

      // Set deprecated to TRUE to hide the target from the UI. This can be
      // useful if you want to rename targets for consistency, but don't want to
      // break importers that are using the old target name.
      // If an importer uses this target it will show up as "DEPRECATED" in the
      // UI.
      'deprecated' => TRUE,
    );
  }

  return $targets;
}

The reset() function just returns the first element of an array. It does not make changes to anything what is in the array.