File format not supported! - Exception raises when creating a Serie or DataFrame with a leading null value 

This code reproduce the error: 

const obj_data = [null, 'bval2', 'bval3', 'bval4'];
const ds = new dfd.Series(obj_data, {dtypes: ['string']});

In production environment its very common to have missing data. 
If data is missing somewhere in the array it seems to work correctly, but in case of first value it raises the exception: File format not supported!

  