`--empty-null` option when importing CSV

[simonw]

CSV files with empty cells in (which come through as the empty string) are common and a bit gross.

Having an option that means "and if it's an empty string store null instead) would be cool.

I brainstormed name options here https://chat.openai.com/share/c947b738-ee7d-419c-af90-bc84e90987da

[simonw]

Documentation at the bottom of this section: https://sqlite-utils.datasette.io/en/latest/cli.html#inserting-csv-or-tsv-data