When a homogeneous block has all values Nothing, only _presentBitset is set. When we apply an operation to such a block using the map method, we decide the size of the block checking the size of _vals and if it is 0 we return an empty block. In the case of all values being Nothing _vals is 0 but the block is not empty.

We should check _presentBitset to decide if the block is empty. If _presentBitset is not zero and _vals is zero then we should return a Monoblock of the same size as the input block and value the result of the operation on Nothing.