prevent php-warnings in visibility, fixes #4598

Closes #4598

Merge request studip/studip!3408

lib/classes/visibility/VisibilitySettings.php

@@ -89,7 +89,7 @@ class VisibilitySettings
      */
     function verify($user_id, $owner_id, $visibility)
     {
-        return $this->states[$visibility] && $this->states[$visibility]->verify($owner_id, $user_id);
+        return !empty($this->states[$visibility]) && $this->states[$visibility]->verify($owner_id, $user_id);
     }
 
     /**
@@ -122,7 +122,7 @@ class VisibilitySettings
 
     /**
      * Returns the number of possible states
-     * @return type 
+     * @return int
      */
     function count()
     {
@@ -130,5 +130,3 @@ class VisibilitySettings
     }
 
 }
-
-?>