Converting file -Invalid data format on input DATE

I am converting data from  dBaseIII format trying to load into Postgresql.

I am having a problem with DATE field, the file I am working with does 
not have any data for
this field on some records.  I have been fixing other problems with sed, 
however I am stumped with this.

Will Postgresql take a null as input for a DATE field?

Does anyone know how to specify the null character in SED?

Thanks in advance.