skip_serializing_if also acts like default

[robin-nitrokey]

Currently, skip_serializing_if not only affects serialization but also deserialization: If a field is marked with this attribute, it is not unwrapped after deserializing. This also means that the attribute only works for options.

[sosthene-nitrokey]

This actually appears to not be the wanted implementation behaviour. It's actually buggy per the serde data model.

Here there should be a: let #ident = #ident.unwrap_or_default().

Right now this works for options because when de-serializing the map, the value is stored in a temporary option, where for a field of type T that option is of type Option<{Inferred Type}>. Where None means that the field is absent, and Some means that the field is present.

The issue is that it doesn't unwrap_or_default, and directly places it in the value. So if T is Option<S> it means that {Inferred type } is inferred to be S. However this is incorrect, the {Inferred Type} should always be T.

This is an issue because:

1. This only works for options with a default to None
2. Even for option this is is incorrect because it will reject the field if it is present but with a value of None. FIDO canonical CBOR appears to allow null values so this could potentially happen, even though it suggests that optional value will be missing, and cbor-smol also handles null values as None.