Explicit null to omit subject (powerline)

[expede]

I chatted with @matheus23 earlier today about the updated powerline syntax & changes from past iterations. [Philipp, feel free to correct me if I'm wrong on any of the below but wanted to capture here.]

TL;DR he's in favour, but raised that it's too easy to omit the sub field accidentally. Two suggestions:

1. Require that you use an explicit sub: null if omitted
2. Use a glob sub: "did:*

The glob has the disadvantage that it looks like you can restrict DIDs by scheme (did:dns:*), when I'm pretty sure that's not the behaviour that we want.

I am in favour of making the Null explicit. In most libraries you're already going to have to give have some variation on audience: Option<DID>, so let's just render None to null as a default.

Any objections?

[Gozala]

It seems consistent with exp so sounds good to me.