message: do not index NULL strings
diff: @@ -55,7 +55,7 @@ static string indexing_script(string input_schema, string input_table, string in
  FROM %input_schema%.%input_table%
  ) AS sq
  WHERE
  - term != ''
  + term != '' AND term NOT NULL
  );
  ALTER TABLE %fts_schema%.docs ADD len INT;
  