Sync: fix issues with PHP 5.3

Problem has been introduced with f885d90

fixes #906

library/Director/Import/Sync.php

@@ -268,9 +268,11 @@ class Sync
                     $filterColumns[$column] = $column;
                 }
             }
+            if (! empty($filterColumns)) {
                 foreach (SyncUtils::getRootVariables($filterColumns) as $column) {
                     $usedColumns[$column] = $column;
                 }
+            }
 
             $rows = $run->fetchRows($usedColumns);
 

library/Director/Import/SyncUtils.php

@@ -125,6 +125,10 @@ class SyncUtils
             }
         }
 
+        if (empty($res)) {
+            return array();
+        }
+
         return array_combine($res, $res);
     }
 }