Fix backwards compatibility for keys with value null

It seems this Pull Request https://github.com/MantleFramework/Mantle/pull/230/files

modified the behaviour of Mantle for keys with a value equals to nil. For example, having a Json as:

{
    "key" : null
}

It was calling the model transformer with "nil", but right now it returns with an Error without calling the transformer.

My fault... my apologize...