channeldb: add ability to store empty auth proof
channeldb: add ability to store empty auth proof

In case if the channel shouldn't be announced to the rest of the network
the proof, which is needed to announce the channel, will not be
populated, fot that reason the ability to store the empty proof has
been added.

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