Fixed non-nullable migration value

Author: DietBepis1

server/db/migrations/20240817143934_create_letter_templates_table.js

@@ -10,7 +10,7 @@ module.exports = {
     })
 
     await knex.schema.alterTable('letters', (table) => {
-      table.integer('letter_template_id').unsigned().notNullable()
+      table.integer('letter_template_id').unsigned()
       table.foreign('letter_template_id').references('letter_templates.id')
       table
         .enu('delivery_method', ['email', 'snail_mail'], {