BUG: Don't return mostly nans from get_history when ffill=False.

Fixes an issue where, if `ffill=False`, `get_history` would return nans for
every entry in the history frame except the last one.

zipline/history/history_container.py

@@ -416,6 +416,8 @@ class HistoryContainer(object):
                     # e.g. with leading nans.
                     pass
             digest_frame = digest_frame.ffill()
+
+        if digest_frame is not None:
             return_frame.ix[:-1] = digest_frame.ix[:]
 
         # Get minutes from our buffer panel to build the last row.