Ignore 'No analyzer warnings!' stdout line from analyzer (#14744)

dev/devicelab/bin/tasks/dartdocs.dart

diff --git a/dev/devicelab/bin/tasks/dartdocs.dart b/dev/devicelab/bin/tasks/dartdocs.dart
index 2beee95..8af47e7 100644
--- a/dev/devicelab/bin/tasks/dartdocs.dart
+++ b/dev/devicelab/bin/tasks/dartdocs.dart
@@ -39,6 +39,8 @@
         // ignore this line
       } else if (entry.startsWith('Analyzing ')) {
         // ignore this line
+      } else if (entry.startsWith('No analyzer warnings!')) {
+        // ignore this line
       } else {
         otherLines += 1;
       }