[Findit] Fix KeyError when no failed targets are extracted.

appengine/findit/waterfall/test/try_job_util_test.py

Index: appengine/findit/waterfall/test/try_job_util_test.py
diff --git a/appengine/findit/waterfall/test/try_job_util_test.py b/appengine/findit/waterfall/test/try_job_util_test.py
index 339550a17bf62aa20d3234080e367792dcbffa70..72a1c76bf8fbf84ce3f43f372905f6d0ff2ea61a 100644
--- a/appengine/findit/waterfall/test/try_job_util_test.py
+++ b/appengine/findit/waterfall/test/try_job_util_test.py
@@ -384,6 +384,10 @@ class TryJobUtilTest(testing.AppengineTestCase):
     self.assertEqual(
         try_job_util._GetFailedTargetsFromSignals({}, 'm', 'b'), [])
 
+    self.assertEqual(
+        try_job_util._GetFailedTargetsFromSignals({'compile': {}}, 'm', 'b'),
+        [])
+
     signals = {
         'compile': {
             'failed_targets': [