Support NULL in mapper

[dhmw]

Ref: shiftcode#285

I noticed that there are types and mappers present in the code base, but the mapper for NULL wasn't being called.

Looking at the mapper code, I see that null/NULL is explicitly caught and bypasses the mapper functions. I believe this is probably an oversight, so I removed it and now I can store and fetch null values.

[myfreax]

request merge. I need it also

[TanjaBayer]

Is there a reason for not merging this? Seems to make sense