[style guide] Add section about preferring undefined to null
[style guide] Add section about preferring undefined to null

Review URL: https://codereview.chromium.org/1824423002

catapult-project/catapult-csm,benschmaus/catapult,catapult-project/catapult,sahiljain/catapult,catapult-project/catapult,sahiljain/catapult,sahiljain/catapult,catapult-project/catapult,benschmaus/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,benschmaus/catapult,sahiljain/catapult,catapult-project/catapult,catapult-project/catapult-csm,catapult-project/catapult,benschmaus/catapult,sahiljain/catapult,catapult-project/catapult,benschmaus/catapult,benschmaus/catapult,catapult-project/catapult-csm,sahiljain/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,catapult-project/catapult,benschmaus/catapult