Fix `bot summary` when URIs couldn't be read.

tools/gardening/bin/summary.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:args/args.dart';
 import 'package:gardening/src/bot.dart';
 import 'package:gardening/src/util.dart';
 
 void help(ArgParser argParser) {
   print('Summarizes the current status of the build bot.');
   print('Usage: summary [options]');
   print('  where options are:');
   print(argParser.usage);
 }
 
 main(List<String> args) async {
   ArgParser argParser = createArgParser();
   ArgResults argResults = argParser.parse(args);
   processArgResults(argResults);
   if (argResults['help']) {
     help(argParser);
     return;
   }
 
   var bot = new Bot(logdog: argResults['logdog']);
   var recentUris = bot.mostRecentUris;
   var results = await bot.readResults(recentUris);
   results.forEach((result) {
-    if (result.hasFailures) {
+    if (result != null && result.hasFailures) {
       print("${result.buildUri} has failures.");
     }
   });
   bot.close();
 }