platform_code optional column

Needed for some feeds, such as Caltrain
https://support.google.com/transitpartners/answer/2450962?hl=en
Skip empty CSV rows

BART’s current GTFS feed contains some empty lines. These rows with
zero elements break namedtuple; skip these.
Check empty strings as floats

Caltrain GTFS includes empty strings in the shape_dist_traveled field,
which fails when trying to convert to float.