Treat missing as optional in UpdateNamespacePropertiesResponse

[mayuriphad]

Closes #4066

Rationale for this change

In the REST spec, UpdateNamespacePropertiesResponse requires only updated and removed. missing is optional and nullable: true ("Server's do not need to implement this"). pyiceberg's response model declared missing: list[str] = Field() as required. So against a server that omits it (Lakekeeper, or anything built on iceberg-rust's response type), RestCatalog.update_namespace_properties raised:

pydantic_core._pydantic_core.ValidationError: 1 validation error for UpdateNamespacePropertiesResponse
missing
  Field required [type=missing, input_value={'updated': [], 'removed': ['a', 'b']}, input_type=dict]

That happens after the server had already applied the update.

missing now defaults to an empty list, and an explicit null is mapped to []. That keeps PropertiesUpdateSummary.missing a list[str] as before, so callers see no type change.

Are these changes tested?

Yes. test_update_namespace_properties_200_without_missing covers a response that omits missing and one with "missing": null. Both fail on main with the ValidationError above and pass with this change. The rest of tests/catalog/test_rest.py passes locally, except test_rest_catalog_with_google_credentials_path, which fails on main too because google-auth isn't installed in my environment.

Are there any user-facing changes?

Yes, a bug fix: update_namespace_properties works with REST servers that omit the optional missing field.

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