Fix missing/unmergeable/empty histograms in value-set-table-cell
Fix missing/unmergeable/empty histograms in value-set-table-cell

Currently, value-set-table throws an exception when the user clicks on a
missing/unmergeable/empty value-set-table-cell.
This CL makes it ignore the click, so it will only display histograms if they
exist and are valid.

BUG=catapult:#2853

Review-Url: https://codereview.chromium.org/2393513002

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