Core: Handle empty tables in RESTCatalogAdapter for server side planning

[geruh]

Summary

This PR fixes scan planning behavior for empty tables in both sync and async modes. While testing the rest fixture, I found that scanning empty tables throws NoSuchPlanIdException. This changes the behavior to return an empty plan instead.

Error:

{
  "error": {
    "message": "Could not find plan ID ccccab5e-9195-422d-b9c0-86b6b6feecad",
    "type": "NoSuchPlanIdException",
    "code": 404,
    "stack": [
      "org.apache.iceberg.exceptions.NoSuchPlanIdException: Could not find plan ID ccccab5e-9195-422d-b9c0-86b6b6feecad",
      "\tat org.apache.iceberg.rest.InMemoryPlanningState.initialScanTasksFor(InMemoryPlanningState.java:116)",
      "\tat org.apache.iceberg.rest.CatalogHandlers.planTableScan(CatalogHandlers.java:675)",
      "\tat org.apache.iceberg.rest.RESTCatalogAdapter.handleRequest(RESTCatalogAdapter.java:311)",
      ....
    ]
  }
}

Testing

Testing with the Docker fixture:

  ./gradlew :iceberg-open-api:shadowJar
  
  docker build -t apache/iceberg-rest-fixture -f docker/iceberg-rest-fixture/Dockerfile .
  
  docker run -d -p 8181:8181 ....

  curl -X POST http://localhost:8181/v1/namespaces/test/tables/empty-tbl/plan \
    -H "Content-Type: application/json" \
    -d '{}'

Returns an empty scan with the fix!

{
  "status": "completed",
  "plan-id": "sync-a1d448e2-c976-4bca-9a02-92a106e535ae",
  "file-scan-tasks": []
}

Note: There aren't any tests for scan planning yet, but those are being added in the client side pr.

cc: @singhpk234 @amogh-jahagirdar @kevinjqliu