Account for null values in veteran history
Account for null values in veteran history

This could use some review from @slifty, but it should fix #52.  I can
now post null values for these fields, save the new client correctly,
and get a 200 response as I should.

PCNI/OpenHMIS,PCNI/OpenHMIS,PCNI/OpenHMIS,PCNI/OpenHMIS