Ensure all created primary keys are not null

[simonw]

Split from:

• Rows with a primary key of null or empty string cannot be edited/deleted/viewed #2805

SQLite allows null primary keys (which then means a table can have multiple rows with the same null primary key value!). Datasette needs to be able to handle those in existing databases - see #2805 - but we can at least stem the bleeding a bit by disallowing them for tables created using the Datasette JSON API.