fix: Handle optional properties in load namespace properties response

[geruh]

Closes #3167

Rationale for this change

The rest spec model GetNamespaceResponse defines the properties field as optional, and nullable. Also, following the description if the rest catalog doesn't support ns properties they should return null.

Link: https://github.com/apache/iceberg/blob/0a73da119ff38ee3a98f248b42180caa51001cec/open-api/rest-catalog-open-api.yaml#L4146-L4163

    GetNamespaceResponse:
      type: object
      required:
        - namespace
      properties:
        namespace:
          $ref: '#/components/schemas/Namespace'
        properties:
          type: object
          description:
            Properties stored on the namespace, if supported by the server.
            If the server does not support namespace properties, it should return null for this field.
            If namespace properties are supported, but none are set, it should return an empty object.
          additionalProperties:
            type: string
          example: { "owner": "Ralph", 'transient_lastDdlTime': '1452120468' }
          default: { }
          nullable: true

Looks like the pydantic models raise a ValidationError in the optional/null cases as seen in the issue above. So this PR adds a fix to handle these cases.

Are these changes tested?

Yes, added tests and tested with s3tables api

Are there any user-facing changes?

Not really

[kevinjqliu]

Suggested change

	properties: Properties = Field(default_factory=dict)
	@field_validator("properties", mode="before")
	@classmethod
	def replace_none_with_dict(cls, v: Any) -> Properties:
	if v is None:
	return {}
	return v
	properties: Properties | None = Field(default_factory=dict)

how about just

[geruh]

This would allow none through to the properties field since the default_factory only applies when the field is ommitted from the response. It would be easier to normalize at the deserialization layer to not miss out on the null checks within the code base but we can.

[kevinjqliu]

i think we actually want to pass null through; for this scenario

        If the server does not support namespace properties, it should return null for this field.

and make load_namespace_properties return both Properties and None

• None for not supported
• Properties for none are set

iceberg-python/pyiceberg/catalog/__init__.py

Line 648 in 4071d39

def load_namespace_properties(self, namespace: str | Identifier) -> Properties:

Wdyt?

[Fokko]

From an user perspective, I think it is more convient to always return a dict