Property access check fails on entity reference fields with empty values

The view access check for an entity reference field property will fail when it should pass given the following conditions:

1. The user has the proper privileges to create a node of a given bundle
2. That bundle has an entity reference field to another node bundle.
3. The user has the proper privileges to view nodes of the referenced bundle.
4. The entity reference field does not have a value.

To reproduce:

// Make sure the user has permission to create articles and view published issues.
global $user;

// Create the article node and get its wrapper.
$values = array('type' => 'article');
$entity = entity_create('node', $values);
$wrapper = entity_metadata_wrapper('node', $entity);

// Check access on an entity reference field without a value.
$access = $wrapper->field_article_issue->access('view', $user);

// $access will always be false.

The above is adapted from https://github.com/RESTful-Drupal/restful/issues/405#issuecomment-102811672. That issue on the restful project appears to be caused by this bug.

This happens because entity_acess() is invoked with a null value for $entity. That in turn calls the hook entity_metadata_no_hook_node_access() which will always return FALSE unless the user has permission to bypass node access or view all nodes.

Perhaps an appropriate solution would be to bypass entityAccess() in EntityDrupalWrapper::access() if the field has no value (and thus no entity to check access against). I'm attaching a patch to that effect.