Added check for ignoreNullValueOverwrites that was being missed.

I thought I put this in the earlier PR but I guess it was missed. 

Just like what Sean instituted [here](https://github.com/vokalinteractive/VOKCoreDataManager/commit/54fe0147ff1cb54c0544cdaa31b7ae84520280b0), this checks for the boolean and value and acts appropriately.