ScalarBuffer::pad_nulls Zeros Out Padded Data Unnecessarily

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

ScalarBuffer::pad_nulls iterates backwards through the values, moving them into their final positions. It does this by swapping them with the zero-initialized value in the destination position.

Describe the solution you'd like

As far as I understand the specification, primitive arrays can have arbitrary values at null positions. Therefore it makes sense, at least to me, to simply copy the value to its destination and not zero out the source position.

Describe alternatives you've considered

The current logic is not incorrect and could remain