code: def test_sharpe_value_when_benchmark_null(self):
      # Sharpe is displayed as '0.0' instead of np.nan
      null_returns = factory.create_returns_from_list(
          [0.0] * 251, self.sim_params
      )
      test_period = ClassicRiskMetrics.risk_metric_period(
          start_session=self.start_session,
          end_session=self.end_session,
          algorithm_returns=null_returns,
          benchmark_returns=null_returns,
          algorithm_leverages=pd.Series(
              0.0, index=self.algo_returns.index
          ),
      )
      self.assertEqual(test_period["sharpe"], 0.0)
  
imports: import datetime
  import pandas
  import numpy
  from zipline.utils import factory
  from zipline.finance.trading import SimulationParameters
  import zipline.testing.fixtures
  from zipline.finance.metrics import _ClassicRiskMetrics
label_desc: Use image node_id 21 for calling the TestRisk obj's underlying member method code with example usage: obj.test_sharpe_value_when_benchmark_null() without return types