Manifest `items` property is optional

[digitaldogsbody]

Expected behaviour:

A Manifest object should not validate if missing the items property - per https://iiif.io/api/presentation/3.0/#34-structural-properties

A Manifest must have the items property with at least one item. Each item must be a Canvas.

Observed behaviour:

You can create a Manifest object without items and Pydantic will not complain

Potential bug location(s):

https://github.com/iiif-prezi/iiif-prezi3/blob/main/iiif_prezi3/skeleton.py#L239

Any other comments:

This is probably a downstream issue from the Validator's JSON Schema. We can temporarily fix it by adding the AutoItems helper to the Manifest class although this only makes sure items is present, not that it has any content.