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

bryanvu/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd,ddustin/lnd,ddustin/lnd,aakselrod/lnd,LightningNetwork/lnd,AndrewSamokhvalov/lnd,LightningNetwork/lnd,bryanvu/lnd