System user was notified when anonymous feedbacks were added

[Son Nguyen]

Version: master (16.04), 15.04, 15.10
Platform: any

Steps:

1. Change email address of user with id=0 in database to your email address.
2. Login as a normal user, create a page and share the page to public allow anonymous feedbacks on it.
3. Logout, access to the page and add a comment

Expected result:
 You should not receive an email with subject "New feedback on ..."

the condition isset($author) should be added in the if statement in the line 1630, htdocs/artefact/comment/lib.php

 if ( isset($author) && !get_field('usr_watchlist_view', 'ctime', 'usr', $author, 'view', $view->get('id')) && ($author != $owner)) {
 ...
 }