fix empty new DataFrame({ series: [] }) handling with automatic index calculation

[nevion]

the range check in utils.range fails when constructing a dataframe where the columns are of 0 length for the automatically calculated default index. Perfectly valid and otherwise working but requires index to be specified every time. This PR adds a trivial fix for it.

I could not figure out how to build danfo as a npmjs.org compatible package - could use some tips there.