[Issue]: minContains on ome.series is inert, so an empty series validates

Describe the issue

schemas/ome.schema constrains series like this:

"series": {
  "description": "An array of the same length and the same order as the images defined in the OME-XML",
  "type": "array",
  "items": {
    "type": "string"
  },
  "minContains": 1
}

In JSON Schema 2020-12, minContains has no effect unless contains is present in the same schema object. There is no contains here, so the keyword is ignored and

{"ome": {"series": [], "version": "0.9.dev1"}}

validates, even though series is required.

The same shape is in the 0.4, 0.5 and 0.6rc0 schemas, and on both main and 0.9dev. By contrast schemas/axes.schema on main pairs minContains with contains, where it does take effect.

Two questions, in order:

1. Should an empty series be valid? The description ties its length to the number of images in the OME-XML, which suggests it should not be.
2. If it should not, minItems: 1 expresses it. That changes which documents validate, so it seemed better to ask here than to fold it into a PR.

Happy to send the change once the first question is settled, or to move the discussion to ome/ngff if the normative half belongs there.

Noticed while vendoring the 0.9.dev1 schemas into ngff-zarr, alongside ome/ngff-spec#195. Related to ome/ngff-spec#194, which asks for JSON linting in the repository: a rule for context-dependent keywords would catch this class of thing.

Start with schemas/ome.schema and compare the series definition across the 0.4, 0.5, 0.6rc0, main, and 0.9dev schemas. Read schemas/axes.schema on main to see the existing contains/minContains pattern, then confirm with maintainers whether empty series is intended to validate. Done means the decision is reflected consistently in the affected schemas and the empty-series behavior is checked.