I want to remove rows where Period End Date is null for the past 20 years.
df, err = ek.get_data(['GOOGL.O', 'MSFT.O', 'FB.O', 'AMZN.O', 'TWTR.K'],
['TR.TotalRevenue.PeriodEndDate', 'TR.TotalRevenue', 'TR.EBITDAActValue', 'TR.EBITActValue'],
{'Scale': 6, 'SDate': -20, 'EDate': 0, 'FRQ': 'FY', 'Curn': 'USD'} )