RowStatisticsOptions: Add null row count

[micdavis]

Is your feature request related to a problem? Please describe.
RowStatisticsOptions has a unique_count option which toggles the calculation of "duplicate_row_count" and "unique row_ratio". But it doesn't have a null_row_count option. We need null_row_count to be an option to toggle to functionality for row_has_null_ratio (link) and row_is_null_ratio (link). This option should be toggled in _update_row_statistics as well to manage the calculation of these metrics (link).

Describe the outcome you'd like:
RowStatisticsOptions needs to include a toggle to enable or disable the calculation for null row count. This option then should be used to the profiler_builder to toggle this functionality.

Additional context:
This is a follow up from current/previous work on RowStatisticsOptions: #841

[taylorfturner]

Closing since the above references PRs were put into dev through staging/dev/options PR #909