Only delete duplicate ids when there are duplicates.

Running a query with an empty IN() is a syntax error

Resources/migrations/VersionAuthserverExternalAccount20171020220239.php

@@ -32,6 +32,7 @@ class VersionAuthserverExternalAccount20171020220239 extends AbstractMigration
         $duplicateIds = array_map(function ($row) {
             return $row['id'];
         }, $duplicates);
+        if (count($duplicateIds) > 0)
             $this->connection->executeUpdate('DELETE FROM vierbergenlars_external_account_external_user WHERE id IN(' . implode(',', $duplicateIds) . ')');
     }