If a wall in unmanned than I believe that wall has a strength value of zero.
This is incorrect. Wall pieces on unmanned wall sections still grant melee strength. However the wall pieces on an unmanned wall can not kill invading units if they have higher combat strength than the invading units at that wall section.
If the invader wins the melee at an unmanned wall section then there will be a breach as there are no defending units to reduce the combat strength difference to zero.