chore: Files feedback_message without NULL state

Column type string should not have a null state to avoid logical
distinction between NULL and '' case.

User defined tests are created without feedback_message but
render a possible feedback_message. Removing the NULL case allows
new records render the empty feedback_message.

An edgecase remains when a test for assessment with a feedback_message
is transformed to a user defined test. The feedback message will
be displayed.

Resolves #3014

Author: arkirchner

db/migrate/20250729085843_remove_null_from_files_feedback_message.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class RemoveNullFromFilesFeedbackMessage < ActiveRecord::Migration[8.0]
+  def up
+    execute <<~SQL
+      UPDATE files SET feedback_message = '' WHERE feedback_message IS NULL;
+    SQL
+
+    change_column_default :files, :feedback_message, ''
+    change_column_null :files, :feedback_message, false
+  end
+
+  def down
+    change_column_null :files, :feedback_message, true
+    change_column_default :files, :feedback_message, nil
+  end
+end