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

tools/gardening/bin/summary.dart

Index: tools/gardening/bin/summary.dart
diff --git a/tools/gardening/bin/summary.dart b/tools/gardening/bin/summary.dart
index 6febcddb8dad3b2a78e9aa7db6ef7fd4a1541121..640f6a62632d4e7611ab04781ab06eed9ebc0c73 100644
--- a/tools/gardening/bin/summary.dart
+++ b/tools/gardening/bin/summary.dart
@@ -26,7 +26,7 @@ main(List<String> args) async {
   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.");
     }
   });