bottom_k should not include nulls if the column contains at least k valid elements

[stinodego]

In discussing #15238, we decided that the behavior for top_k/bottom_k with regards to nulls should be:

1. Valid elements have priority over null values: if a column contains at least k valid elements, the result will not contain any nulls
2. If the column contains fewer than k valid elements, nulls from the original column are used to reach k resulting elements.
3. If k is larger than the size of the column, the result will be all the elements of the original column. We do not 'pad' the result with nulls until we have k elements.

This is currently the case for top_k, but not for bottom_k. This behavior should be updated for 1.0.0.