docstr: Test _proto_to_dict() to deserialize protobuf with optional field
code: def test_optional_field(self):
      """Test _proto_to_dict() to deserialize protobuf with optional field"""
      min_replicas = 1
      proto = AutoscalingConfigProto(
          min_replicas=min_replicas,
      )
      result = _proto_to_dict(proto)
  
      # Non-empty field is filled correctly.
      assert result["min_replicas"] == 1
  
      # Empty field is filled correctly.
      assert result["max_replicas"] == 0
  
      # Optional field should not be filled.
      assert "initial_replicas" not in result
  
imports: import pytest
  from ray import cloudpickle
  from ray._private.pydantic_compat import ValidationError
  from ray._private.utils import import_attr
  from ray.serve._private.config import DeploymentConfig, ReplicaConfig, _proto_to_dict
  from ray.serve._private.constants import DEFAULT_AUTOSCALING_POLICY, DEFAULT_GRPC_PORT
  from ray.serve._private.utils import DEFAULT
  from ray.serve.autoscaling_policy import default_autoscaling_policy
  from ray.serve.config import AutoscalingConfig, DeploymentMode, HTTPOptions, ProxyLocation, gRPCOptions
  from ray.serve.generated.serve_pb2 import AutoscalingConfig
  from ray.serve.generated.serve_pb2 import DeploymentConfig
  from ray.serve.generated.serve_pb2 import DeploymentLanguage
  from ray.serve.generated.serve_pb2_grpc import add_UserDefinedServiceServicer_to_server
  from ray.serve.schema import DeploymentSchema, HTTPOptionsSchema, ServeApplicationSchema, ServeDeploySchema
label_desc: Use image node_id 6 for calling the TestProtoToDict obj's underlying member method code with example usage: obj.test_optional_field() without return types