utxonursery: handle non-existant to-self outputs
utxonursery: handle non-existant to-self outputs

If a forceClose happens and the to-self output is below the dust value,
then that output will be non-existant in the commitment transaction.
UtxoNursery now handles that case by checking to see if the accompanying
selfOutPutSignDescriptor is nil. If it is nil, then it will ignore it.

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