Deserialize undefined as None for Option

[mraerino]

CBOR knows these two types (spec):

• 0xf6 => null
• 0xf7 => undefined

Serde only knows Null afaik, or at least the Value enum doesn't have anything closer to represent undefined than Null.

In order for this to work well with Option we need to treat undefined the same as null when deserializing an Option.

this popped up when interacting between Rust and JS, when using { key: undefined } in JS and reading it into struct { key: Option<Foo> }