test(sdk): pin the empty-label contender path (None → null label, contender kept) #5752

Expected Behavior

Tests pin what happens to a contender whose domain document has an empty label:

• ContenderRequest reads it as label: None and keeps created_at;
• the C contender built from it has a null label pointer, and has_created_at / created_at stay as decoded;
• the contender stays in the list. It is not dropped, and only its display label is missing.

Current Behavior

#5718 moved the contender decode into rs-sdk. ContenderRequest::from_document filters an empty label to None, and ffi_contender turns None into a null pointer. The list conversion used to allocate an empty C string instead. No test covers the empty-label case.

A valid DPNS document cannot have an empty label: DPNS v1–v3 require minLength: 3, and Drive-ABCI enforces it when the document is created. The tests are defensive. Null already means "did not decode", and the Swift readers already handle it.

Possible Solution

• rs-sdk: a unit test in contender_request.rs that builds a domain document with label: "" and asserts label == None and created_at unchanged.
• rs-sdk-ffi: a test in contender_document.rs that passes a ContenderRequest with label: None to ffi_contender and asserts a null label, the timestamp fields as given, and the identity id kept. It frees the contender afterwards.

Alternatives Considered

Reject empty labels explicitly in rs-sdk-ffi. That check already happens in rs-sdk, the layer that owns the decode, so a second check in the FFI would duplicate it.

Additional Context

Raised in review of #5718: #5718 (comment)