Don't show exception when no announcement has been created

refs #11198

library/Icinga/Web/Announcement/AnnouncementIniRepository.php

@@ -117,10 +117,13 @@ class AnnouncementIniRepository extends IniRepository
     public function getEtag()
     {
         $file = $this->getDataSource('announcement')->getConfigFile();
+        if (@is_readable($file)) {
             $mtime = filemtime($file);
             $size = filesize($file);
             return hash('crc32', $mtime . $size);
         }
+        return null;
+    }
 
     /**
      * Get the query for all active announcements