Handle nulls when reading (didn't test writing)

[nugend]

trafficstars

The library is not respecting the null bitmaps. These could probably be handled using the Compute API to fill in the null KDB values.

[nugend]

Before Arrow 6.0, this can be done with the fill_null compute function, 6.0 and greater, it needs to be done with the coalesce compute function.

[vgrechin-kx]

Delivered in Release 1.2.0