csv/root/yoda: allow for columns with missing values

[GraemeWatt]

For example, Tables 2, 3, 4, 7 and 8 of https://www.hepdata.net/record/ins1216885 return an HTML file with IndexError: list index out of range when attempting to convert to CSV, ROOT or YODA formats. The URL https://www.hepdata.net/download/submission/ins1216885/1/root currently appears on the first page of results in a Google search for return self.wsgi_app(environ, start_response). This came up in "Top growing queries" for May 2022 in an email from Google Search Console.