[Android] Only remember non-None iteration results.

https://codereview.chromium.org/1376483002 broke trigger-only AMP calls
to build/android/test_runner.py. This fixes that workflow.

BUG=
TBR=mikecase@chromium.org

Review URL: https://codereview.chromium.org/1371243002

Cr-Commit-Position: refs/heads/master@{#351151}

build/android/test_runner.py

@@ -959,9 +959,9 @@ def RunTestsInPlatformMode(args, parser):
                        else itertools.count())
         for _ in repetitions:
           iteration_results = test_run.RunTests()
-          results.append(iteration_results)
 
           if iteration_results is not None:
+            results.append(iteration_results)
             report_results.LogFull(
                 results=iteration_results,
                 test_type=test.TestType(),