Add not-null modifier to migrations

[dhh]

# Generating with...
bin/rails generate migration CreateUsers email_address:string!:uniq password_digest:string!

# Produces:
class CreateUsers < ActiveRecord::Migration[8.0]
  def change
    create_table :users do |t|
      t.string :email_address, null: false
      t.string :password_digest, null: false

      t.timestamps
    end
    add_index :users, :email_address, unique: true
  end
end

[dhh]

Unrelated test failures.