fix(ffi): guard empty receipt arrays

[Andiveli]

Closes #381

Summary

• encode empty receipt arrays as a coherent nil/zero ABI pair
• decode empty arrays without constructing a Rust slice from a null pointer
• preserve non-empty callback lifetime and ABI layout

Chain context

#377 fix/ffi-jid-ownership
  └── 📍 fix/ffi-receipt-empty-results

Base: fix/ffi-jid-ownership
Depends on: #377
Follow-up: combined beta validation after both PRs pass.

Test plan

• Focused and full Go tests passed
• Rust bridge tests — 27 passed
• Diff checks passed