code: def test_th_improvement_rate__optimized_model_is_none(self):
      res = OptimizeInferenceResult(
          original_model=MagicMock(),
          hardware_setup=MagicMock(),
          optimized_model=None,
      )
      self.assertIsNone(res.throughput_improvement_rate)
  
imports: import unittest
  from unittest.mock import MagicMock
  from nebullvm.core.models import OptimizeInferenceResult
label_desc: Use image node_id 6 for calling the TestOptimizeInferenceResult obj's underlying member method code with example usage: obj.test_th_improvement_rate__optimized_model_is_none() without return types