_features_sanitize does not always remove 'empty' values

This of course depends on the definition of 'empty' :)

But I have field settings that keep giving differences, and this is what I saw:

  $default = array(
    'name' => 'name',
    'settings' => array(),
  );

  $normal = array(
    'name' => 'name',
    'settings' => array(
      'empty_value' => NULL,
    ),
  );

_features_sanitize removes the 'settings' key from $default.

But it does not remove the 'settings' key from $normal; it leaves 'settings => array()' in there. I'm guessing that this is not as desired.

Patch attached.