sidekick: swift supports `null` as default values

When decoding from ProtoJSON we need to treat `null` as the default value. Note that this works for required, optional, repeated, and map fields:

- For required numeric fields `null` is decoded as `0`
- For required string fields `null` is coded as `""`
- For required bytes fields `null` is decoded as the empty `Data()`.
- For required enum fields `null` is decoded as the default enum value in the enum (there is always one in proto3).
- For optional fields `null` is decoded as `nil`
- For repeated fields `null` is decoded as an empty array.
- For map fields `null` is decoded as an empty dictionary.