Summary statistics for records with NULL values

The summary statistics for a given metric will be incorrect if any logbook records have NULL value for that metric (frequency, band, mode, etc.)

To avoid this, if any records have NULL values for a given metric they will be added to the "Unknown" count.

For example if there are 20 records with NULL values for the mode, then the count of the the "Unknown" in the modes summary table will be 20, and the percentage of the total will be displayed. This should be better than simply ignoring these records from the total in the summaries.