Add a missing consensus rule for v4 transactions: if there are no Sapling spends or outputs, valueBalance MUST be 0.
Add a missing consensus rule for v4 transactions: if there are no Sapling spends or outputs, valueBalance MUST be 0.

Signed-off-by: Daira Hopwood <cd7d43d5556a38ce626d53826fddc46adc597539@jacaranda.org>

zcash/zips,zcash/zips,zcash/zips,zcash/zips