CQ master should ignore NoneType failure messages in _GetInfraFailMessages
CQ master should ignore NoneType failure messages in _GetInfraFailMessages

The failed ToT sanity check builders returns None messages because
they do not apply patches. CQ master should ignore such failed
messages.

BUG=chromium:380921
TEST=`cbuildbot/run_tests`

Change-Id: I89823862f4db431b90cd3ce9ea280cba0f846db4
Reviewed-on: https://chromium-review.googlesource.com/202613
Reviewed-by: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>
Commit-Queue: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>
Tested-by: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>

bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite