gather_builder_stats: tolerate empty good_patch_rejections
gather_builder_stats: tolerate empty good_patch_rejections

BUG=None
TEST=None

Change-Id: Ide74db494cb7c7d6ff986497b71349bacec22ba1
Reviewed-on: https://chromium-review.googlesource.com/197506
Reviewed-by: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>
Tested-by: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>

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