A dead voltage sense line now reads as missing, not as zero production

A combiner box behind a data logger previously published a measured 0 W while its strings carried real current. A missing channel now publishes nothing, and an explicit zero still reads as zero. Plants receive this with their next agent update.

• A box whose string sum is incomplete now shows no box total until every string answers again.
• The fix covers the box's power, current and temperature readings.