BUG: Explicitly guard against empty bar data in history container

contstruction.

BarData can be falsey. in create_buffer_panel, the intention of the
check against bar_data was to see if it was passed at all, not if it was
truthey. In order to make that check more explicit, the check now
asserts that bar_data is not None.

tests/test_history.py

@@ -22,7 +22,7 @@ import numpy as np
 import pandas as pd
 from pandas.util.testing import assert_frame_equal
 
-from zipline.history import history, Frequency
+from zipline.history import history
 from zipline.history.history_container import HistoryContainer
 from zipline.protocol import BarData
 import zipline.utils.factory as factory

zipline/history/history_container.py

@@ -594,7 +594,7 @@ class HistoryContainer(object):
         )
         self.buffer_spec = spec
 
-        if bar_data:
+        if bar_data is not None:
             frame = self.frame_from_bardata(bar_data, initial_dt)
             rp.add_frame(initial_dt, frame)