Don’t rely solely on has_validity to determine whether a series contains nulls

From the Python documentation for has_validity (emphasis mine):

While the absence of a validity bitmask guarantees that a Series does not have null values, the converse is not true, eg: the presence of a bitmask does not mean that there are null values, as every value of the bitmask could be false.

To confirm that a column has null values use null_count().

This PR modifies the logic for Series.toTypedArray (in line with the note quoted above) to check nullCount() if the Series hasValidity(), rather than relying on the hasValidity() check alone.

Closes #206