fix(rest): omit CreateTableRequest.location when None

[forrestlinfeng]

Which issue does this PR close?

No issue filed — reporting and fixing directly. Happy to open one if preferred.

What changes are included in this PR?

CreateTableRequest.location is declared as Option<String> but lacks skip_serializing_if, so serde_json emits "location": null when the caller passes None. Per the Iceberg REST spec the field is optional and should be omitted when unset.

Several REST-compatible catalog servers (notably AWS Glue REST) reject a present-but-null location and never fall back to a server-side default (e.g. the Glue database's LocationUri). The result is Location information cannot be null errors that cannot be resolved from the server side — the database may already have a LocationUri set, but it's never consulted because the request actively provides location: null.

This PR adds #[serde(default, skip_serializing_if = "Option::is_none")] to the location field, matching how properties is already handled on the same struct and matching spec semantics for optional fields.

     /// Optional table location. If not provided, the server will choose a location.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub location: Option<String>,

Scope

Intentionally minimal — only location is touched. The same Option<T> pattern without skip_serializing_if exists on partition_spec, write_order, and stage_create, and is likely worth the same treatment in a follow-up, but those haven't been observed to trigger server-side rejections in practice.

Compatibility

This is a backwards-compatible serialization change:

• Before: {"name": "t", "location": null, "schema": {...}, ...}
• After: {"name": "t", "schema": {...}, ...}

Spec-compliant REST servers MUST treat a missing optional field the same as an explicit null, so well-behaved servers (Polaris, Tabular, Lakekeeper, etc.) are unaffected. Strict servers like Glue REST start working.

Are these changes tested?

Yes — added a unit test types::tests::test_create_table_request_omits_null_location that constructs a CreateTableRequest { location: None, .. } and asserts the serialized JSON has no "location" key. Existing catalog::tests::test_create_table and test_create_table_409 still pass.

$ cargo test -p iceberg-catalog-rest --lib types:: catalog::tests::test_create_table
running 2 tests
test types::tests::test_create_table_request_omits_null_location ... ok
test types::tests::test_namespace_response_serde ... ok
test result: ok. 2 passed; 0 failed
running 2 tests
test catalog::tests::test_create_table_409 ... ok
test catalog::tests::test_create_table ... ok
test result: ok. 2 passed; 0 failed