Fix unique validator (#288)
Fix unique validator (#288)

Shouldn't check undefined and null values, since ORM uses these as
defaults and it can lead to failed validations regardless of passed
values.

benkitzelman/node-orm2,jtrinc/node-orm2,hallta/node-orm2,kapouer/node-orm2,kolonse/node-orm2,benkitzelman/node-orm2,ScoutGroup/onionrm,kolonse/node-orm2,locomote/node-orm2,mariamjpinto/node-orm2,kapouer/node-orm2,dresende/node-orm2,kolonse/node-orm2,benkitzelman/node-orm2,pskucherov/node-orm2,kapouer/node-orm2,stueynz/node-orm2