[Bug]: Jaeger Agent's http /sampling endpoint returns JSON which breaks older jaeger clients

[vprithvi]

What happened?

Jaeger Agent's sampling endpoint's perOperationStrategies field returns null instead of [], which breaks older Java jaeger-clients.

Steps to reproduce

As of 1.23, sampling endpoint returns the following when using grpc as transport between agent and collector.

$ curl "localhost:5778/sampling?service=none" | jq
 
  "strategyType": "PROBABILISTIC",
  "probabilisticSampling": {
    "samplingRate": 0
  },
  "operationSampling": {
    "defaultSamplingProbability": 0.001,
    "defaultLowerBoundTracesPerSecond": 1e-05,
    "perOperationStrategies": null
  }
}

Expected behavior

As of version 1.17, sampling endpoint returns the following when using tchannel as transport between agent and collector

$ curl "localhost:5778/sampling?service=none" | jq
 
  "strategyType": "PROBABILISTIC",
  "probabilisticSampling": {
    "samplingRate": 0
  },
  "operationSampling": {
    "defaultSamplingProbability": 0.001,
    "defaultLowerBoundTracesPerSecond": 1e-05,
    "perOperationStrategies": []
  }
}

This is because the grpc/proto response from collector uses nil as the default value for slice, and json.Marshal then encodes nil as null. It is possible that tchannel/thrift didn't follow the same conventions and used empty arrays instead.

From https://pkg.go.dev/encoding/json#Marshal,

Array and slice values encode as JSON arrays, except that []byte encodes as a base64-encoded string, and a nil slice encodes as the null JSON value.