Tolerate invalid date format in shapefile attribute (NULL).
Tolerate invalid date format in shapefile attribute (NULL).

NULL values are usually encoded as '00000000', this commit handles
any invalid date turning it to a value=0.

Fixes the ValueError('year is out of range',) message.

datapolitan/cartodb20,datapolitan/cartodb20,jmwenda/nationmaps,datapolitan/cartodb20,jmwenda/nationmaps,datapolitan/cartodb20,jmwenda/nationmaps,jmwenda/nationmaps