Constructor: Create empty TSFrame if an empty DataFrame is provided

[chiraganand]

There is a constructor for creating an empty TSFrame object but currently it does not work with a DataFrame without any columns. Ideally, TSFrame(DataFrame()) should create a TSFrame with an Index column and 0 other non-index columns and 0 rows.

 julia> TSFrame(DataFrame())
 0×0 TSFrame with Int64 Index