spec should clarify when meta should be omitted

[Gozala]

Current description of meta field introduces ambiguity and it would be good to address it

delegation/README.md

Line 61 in e108236

| `meta` | `{String : Any}` | No | [Meta] (asserted, signed data) — is not delegated authority |

Spec says meta is not required, which makes it not clear what to do in case of empty map {} should it be encoded as null or as empty object.

This is problematic because different implementations may choose different defaults e.g. iso-ucan seems to encode it as {} where's some other implementation e.g. rs-ucan may choose to encode it as null which would mean that decoding and encoding back will not roundtrip.

I don't think rs-ucan actually does that, but it had some other unrelated issues that made me thing this was happening.

I suggest to update spec so that meta is either required or make spec require for empty map to be represented an null so implementations can be consistent.