fix split vector layer tool (QPyNullVariant again!)

I am seeing something strange with ```QPyNullVariant``` by using some fTools tool. if there is a ```NULL``` value in attribute table ```QgsFeature::attributeMap()``` returns ```None```  so some tool may not work as expected, while in 1.8 it returns an empty ```QString``` and the tool works properly also with ```NULL``` value.

This patch allows to write a feature even when some attribute value is ```NULL``` (or ```QPyNullVariant```).

I am no sure if it is correct, but I had to insert a check on ```atMap[index]```, which returns ```None``` if the value is ```NULL```, to add the feature to ```QgsVectorFileWriter```

Maybe I am wrong, but we have to find a way to fix this in core, is it possible ?