message: tolerate null genertions
diff: @@ -115,7 +115,7 @@ module.exports = class PgClient {
  }
  async deleteOlderGenerations(branch, sourceId, nonce) {
  - let sql = 'delete from documents where generation!=$1 and source=$2 and branch=$3';
  + let sql = 'delete from documents where (generation != $1 or generation is null) and source=$2 and branch=$3';
  await this.query(sql, [nonce, sourceId, branch]);
  }
  