Description:
    Generates two new database migrations which will safely add a check that
    a column is not null, and validate it separately.

    If no --database option is specified, uses the default database.

Example:
    rails generate nandi:not_null_check foos bar

    This will create:
        db/safe_migrations/20190424123727_add_not_null_check_on_bar_to_foos.rb
        db/safe_migrations/20190424123728_validate_not_null_check_on_bar_to_foos.rb

Example:
    rails generate nandi:not_null_check foos bar --validation-timeout 20000

    This will create:
        db/safe_migrations/20190424123727_add_not_null_check_on_bar_to_foos.rb
        db/safe_migrations/20190424123728_validate_not_null_check_on_bar_to_foos.rb

    The statement timeout in the second migration will be set to 20,000ms.

Multi-database examples:
    rails generate nandi:not_null_check users email --database primary
        creates not null check migrations in primary database directory

    rails generate nandi:not_null_check events user_id --database analytics
        creates not null check migrations in analytics database directory
