Add support for optional fields with null serialised

This follows a conversation we had for how to deal with
optional-but-required fields in JSON serialisations: fields which are
optional (can be either a given type or 'null'), but where the 'null'
value is required. There are just a few of these in the Python code,
but we should support them nevertheless.

The patch changes the 'isOptional' attribute from boolean to a custom
ADT, three-typed. This allows us to keep the same path on load (which
deals with both cases), but use a custom save path where we explicitly
save the 'null' value.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>