Fix notice in PhraseModel, a=chris

diff --git a/src/models/PhraseModel.php b/src/models/PhraseModel.php
index ad25b2824..3b52c6113 100755
--- a/src/models/PhraseModel.php
+++ b/src/models/PhraseModel.php
@@ -1306,8 +1306,10 @@ class PhraseModel extends ParallelModel
                     }
                 }
             }
-            L\orderCallback($pages[0], $pages[0], self::SCORE);
-            usort($pages, C\NS_LIB . "orderCallback");
+            if (!empty($pages)) {
+                L\orderCallback($pages[0], $pages[0], self::SCORE);
+                usort($pages, C\NS_LIB . "orderCallback");
+            }
             $sort_time = L\changeInMicrotime($sort_start);
         }
         if ($num_retrieved < $to_retrieve) {