Ensure serialisation correctly respects required and nullable fields

[crbunney]

The EDR OpenAPI schema identifies some elements as required and/or nullable

We haven't been particularly rigorous in adhering to these restrictions in our initial implementation.

The implications for the core.models data models:

• it should be possible to set nullable values to None
• values for optional fields should default to None if not explicitly set
• values that are not allowed to be nullable should be always set, either by an explicit argument or sensible default

The implications for serialisation are:

• when encoding nullable fields (both optional and required), None values are serialised as JSON null values
• when encoding non-nullable, optional fields, the field is omitted from the output if the value is None
• non-nullable, required fields set to None would be an error, and ought not to be possible if our core.models are correctly implemented

I believe the possible options are something like

	nullable=False	nullable=True
required=True	None not allowed in data models; always included in serialisation	None allowed in data models; always included in serialisation
required=False	None allowed in data models; not included in serialisation if set to None	None allowed in data models; Element omitted from serialisation if set to None