sequelize/sequelize — pull request #572 by mweibel (2013-04-29 13:02:29+00:00)
Title: When value is null, don't try to convert to Date, instead just leave it

I ran into a problem when I have allowed null on a Date-Field and if I tried to match against
null it didn't work. Instead a Date with "Invalid Date" has been put into this field.

I don't know if there needs to be some additional checks, but I think it should work out.