Make L2pL1_dual resilient to empty SBar.

When we are screening out variables, we remove them
from the SBar support explored during optimization.
During the optimization, we may compute a dual objective value.
This dual depends on SBar.
The edge case of empty SBar wasn't taken into account before,
leading to an exception when taking some max of an empty vector.
This is now fixed: no SBar = don't take that maximum.