Transaction staging: don't assume an identity exists (ie, allow staging of genesis transaction)

[orthecreedence]

In a few places, we assume that when staging transactions we'll have an identity ID. This holds true not just for the commands in the CLI but also the aux library's stage functions.

This assumption is false, because it's entirely plausible and acceptable to use the staging system to create a multiparty group identity. This needs to be fixed.

That said, having all staged transactions in one big pool seems like a step backward in terms of UX, so maybe assigning an identity_id if we have it, but allowing null if we don't is a good compromise, and in the cli making the identity id optional, or add an argument for "has no identity id."