fix: Handle optional properties in load namespace properties response (#3169)

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

```yaml
    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

Author: geruh

pyiceberg/catalog/rest/__init__.py

@@ -337,7 +337,14 @@ class ListNamespaceResponse(IcebergBaseModel):
 
 class NamespaceResponse(IcebergBaseModel):
     namespace: Identifier = Field()
-    properties: Properties = Field()
+    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
 
 
 class UpdateNamespacePropertiesResponse(IcebergBaseModel):

tests/catalog/test_rest.py

@@ -922,6 +922,39 @@ def test_load_namespace_properties_200(rest_mock: Mocker) -> None:
     assert RestCatalog("rest", uri=TEST_URI, token=TEST_TOKEN).load_namespace_properties(namespace) == {"prop": "yes"}
 
 
+def test_load_namespace_properties_200_without_properties(rest_mock: Mocker) -> None:
+    namespace = "leden"
+    rest_mock.get(
+        f"{TEST_URI}v1/namespaces/{namespace}",
+        json={"namespace": ["leden"]},
+        status_code=200,
+        request_headers=TEST_HEADERS,
+    )
+    assert RestCatalog("rest", uri=TEST_URI, token=TEST_TOKEN).load_namespace_properties(namespace) == {}
+
+
+def test_load_namespace_properties_200_with_null_properties(rest_mock: Mocker) -> None:
+    namespace = "leden"
+    rest_mock.get(
+        f"{TEST_URI}v1/namespaces/{namespace}",
+        json={"namespace": ["leden"], "properties": None},
+        status_code=200,
+        request_headers=TEST_HEADERS,
+    )
+    assert RestCatalog("rest", uri=TEST_URI, token=TEST_TOKEN).load_namespace_properties(namespace) == {}
+
+
+def test_load_namespace_properties_200_with_empty_properties(rest_mock: Mocker) -> None:
+    namespace = "leden"
+    rest_mock.get(
+        f"{TEST_URI}v1/namespaces/{namespace}",
+        json={"namespace": ["leden"], "properties": {}},
+        status_code=200,
+        request_headers=TEST_HEADERS,
+    )
+    assert RestCatalog("rest", uri=TEST_URI, token=TEST_TOKEN).load_namespace_properties(namespace) == {}
+
+
 def test_load_namespace_properties_404(rest_mock: Mocker) -> None:
     namespace = "leden"
     rest_mock.get(