Revisit default values for list query parameters/fields

[alyssadai]

In our data models, several fields/parameters that accept lists are defined with a default value of None, e.g.:

federation-api/app/api/models.py

Lines 49 to 53 in 485b33d

	class NodeDatasets(BaseModel):
	"""Data model for specifying datasets to query within a specific node."""
	node_url: str
	dataset_uuids: list[str] | None = None

When these fields are optional, it means that the API will have the following parsing behaviour:

• A request where this field isn't included at all (undefined) --> dataset_uuids=None
• "dataset_uuids": null -> dataset_uuids=None
• "dataset_uuids": [] --> dataset_uuids=[]

Right now we treat [] and None the same way for these fields, but the fact that what's considered an empty value can be ambiguous is probably not ideal. A better practice may be to default to an empty list [] so that null values are disallowed, so that we avoid having to guess in the future if the field is [] or None.

[surchs]

@alyssadai can you provide more context on this?

[alyssadai]

Added - let me know if anything is unclear about the description!

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