Document behavior if a field / key is missing

[birdsarah]

The behavior, will be the default json schema behavior - if its missing it'll be null or the default value: http://json-schema.org/latest/json-schema-validation.html#anchor10

[practicalparticipation]

The only default we currently have in the schema is for language, and I've suggested this should be removed, with language always explicitly set.

The draft documentation I have for this is as follows:

Basics (within Schema Reference)

If a particular field is not used within a given release or record, that field can be ommitted from the serialised data. Systems consuming the data can record a null value for this field.

To minimise file sizes publishers may omit fields they are not using.

However, publishers should note that including a field, and explicitly setting its value to ‘null’ is meaningful for the merging of releases into records, as documented in the section on merging (See draft documentation under #42).

[practicalparticipation]

(Note - a code update to the mapper might be required so that it does not output null fields, to avoid upsetting merges if we take this approach).

[birdsarah]

@practicalparticipation I'm closing this as you've got this covered in the docs you wrote up for #42.