utxonursery: ensure we don't attempt to create negative value'd outputs

This commit fixes a slight bug in the utxoNursery. Previously, if we
forced closed a channel that was solely funded by the other party
without pushing any satoshis, then the utxoNursery would attempt to
sweep a target output even though it didn’t actually exist. This would
result in the creation of a negative value’d output due to the hard
coded fees currently used in several areas.

To fix his, we now ignore any “output” with a value of zero, when
adding new outputs to the kindergarden bucket.

utxonursery.go

@@ -314,6 +314,13 @@ out:
 				len(preschoolRequest.outputs))
 
 			for _, output := range preschoolRequest.outputs {
+				// We'll skip any zero value'd outputs as this
+				// indicates we don't have a settled balance
+				// within the commitment transaction.
+				if output.amt == 0 {
+					continue
+				}
+
 				sourceTxid := output.outPoint.Hash
 
 				if err := output.enterPreschool(u.db); err != nil {