Enable differential.find handle empty input.

Summary:
return empty array when the query input is empty.

Test Plan:
send empty input.

Reviewed By: tuomaspelkonen
Reviewers: tuomaspelkonen
CC: tuomaspelkonen
Differential Revision: 167

src/applications/conduit/method/differential/find/ConduitAPI_differential_find_Method.php

@@ -51,12 +51,16 @@ class ConduitAPI_differential_find_Method extends ConduitAPIMethod {
     $query = $request->getValue('query');
     $guids = $request->getValue('guids');
 
+    $results = array();
+    if (!$guids) {
+      return $results;
+    }
+
     $revisions = id(new DifferentialRevisionListData(
       $query,
       (array)$guids))
       ->loadRevisions();
 
-    $results = array();
     foreach ($revisions as $revision) {
       $diff = $revision->loadActiveDiff();
       if (!$diff) {