energy: Fix null harvester issue in EnergySource
This MR fixes an issue in which the calculated total current becomes a NaN (See EnergySource::CalculateTotalCurrent
).
This is caused by the pointer to EnergyHarvester
which becomes invalid when no harvesters are installed into the
EnergySource
. A simple check of whether or not EnergyHarvesters are associated with the EnergySource fixes this issue.