Fix the handling of to_dataframe when the table is empty.
Fix the handling of to_dataframe when the table is empty.

Issue #685

googledatalab/datalab,googledatalab/datalab,craigcitro/datalab,parthea/datalab,yebrahim/datalab,parthea/datalab,yebrahim/datalab,craigcitro/datalab,yebrahim/datalab,yebrahim/datalab,parthea/datalab,googledatalab/datalab,craigcitro/datalab,craigcitro/datalab,parthea/datalab,yebrahim/datalab,parthea/datalab,craigcitro/datalab,googledatalab/datalab,googledatalab/datalab,parthea/datalab,googledatalab/datalab