Idea: Allow destination to be empty

[farscapian]

Currently we require that the destination be a valid BOLT12 offer. However, we COULD allow it to be an emptry string. Of course, no payouts will occur until a valid BOLT12 offer exists in the destination. But allowing empty strings allows us to have placeholders that accrue outlays. A good example is someone who deserves credit for something and thus should be in the prism, but cannot yet produce a BOLT12 offer.