When viewing a thread with no picture, load no images instead of all images

Summary:
Currently, we select every file when looking at a thread with no splash image.

(Possibly the Query stuff should try to catch this.)

Test Plan: No more `SELECT * FROM files` query.

Reviewers: chad, btrahan, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4887

src/applications/conpherence/query/ConpherenceThreadQuery.php

@@ -227,6 +227,11 @@ final class ConpherenceThreadQuery
         $conpherence_pic_phids[$conpherence->getPHID()] = $phid;
       }
     }
+
+    if (!$conpherence_pic_phids) {
+      return $this;
+    }
+
     $files = id(new PhabricatorFileQuery())
       ->setViewer($this->getViewer())
       ->withPHIDs($conpherence_pic_phids)