code: def testNullPortfolio(self):
      retTracker = returns.TimeWeightedReturns(0)
      self.assertEqual(retTracker.getCumulativeReturns(), 0)
  
imports: import datetime
  from .None import common
  from .None import strategy_test
  from .None import position_test
  from testcases import test_strategy
  from pyalgotrade.barfeed import yahoofeed
  from pyalgotrade.barfeed import csvfeed
  from pyalgotrade.stratanalyzer import returns
  from pyalgotrade import broker
  from pyalgotrade import marketsession
label_desc: Use image node_id 1 for calling the TimeWeightedReturnsTestCase obj's underlying member method code with example usage: obj.testNullPortfolio() without return types