Small fixes to create Items out of RDPS and HRDPS data sets

[huard]

RDPS and HRDPS are the datasets used to train the spatial downscaling U-Net in the GeoConnexions project.

There are small test samples here:
https://pavics.ouranos.ca/twitcher/ows/proxy/thredds/catalog/birdhouse/testdata/HRDPS/catalog.html

This PR runs STAC item creation tests against those datasets. The only fixes needed to run the tests were

1. Add a default create_uid method in the THREDDSCatalogDataModel. Not doing so would mean having to create a custom extension for every dataset we want to add to the catalog. The default UID is just the location of the dataset relative to the fileserver/ URL.
2. Change how empty vertical spatial extents are encoded to match the schema. Previously [", "], now [None, None].

Notes

In the DataCube 2.2.0 schema, the vertical_spatial_dimension has a required extent defined as

extent_open": {
      "type": "array",
      "minItems": 2,
      "maxItems": 2,
      "items": {
        "type": [
          "number",
          "null"
        ]
      }
    }

[fmigneault]

Needs a run of ruff format.