fix(ai-summary): use empty lists when the backend responds with nulls

[sseu-buhzzi]

The backend can responds with:

{
  code: 1001,
  message: "processing",
  data: {
    hole_id: 0,
    trace_id: "",
    summary: "",
    branches: null,
    interactions: null,
    keywords: null,
    generated_at: "",
  },
}

Therefore default values are needed for backend-nullable fields. Or as another fix, the backend can return data as null or {}.