Fixed broken rollback migratio in processes' name

This is a migration fix added by 76d2c0f.
Since `up` is a change to NOT NULL, `down` must undo it.

Author: tmimura39

db/migrate/20240813160053_make_name_not_null.rb

@@ -11,6 +11,6 @@ def up
 
   def down
     remove_index :solid_queue_processes, [ :name, :supervisor_id ]
-    change_column :solid_queue_processes, :name, :string, null: false
+    change_column :solid_queue_processes, :name, :string, null: true
   end
 end