Canonicalise empty env/plugins/matrix to nil for sign/verify

[DrJosh9000]

jcs does not canonicalise {} and [] to null (or vice-versa).

encoding/json would obey omitempty as a directive in a struct field tag, but the canonicalPayload isn't encoding Values as a struct - it's encoding a map[string]any.

Since there are a bunch of signatures floating about now, I made an educated guess that null would be the most common value for un-set env, plugins, and matrix. But this may still break some signatures.

[wolfeidau]

👍🏻 🚀 NICE, seems like a good move to make this more consistent!