Fix start_date validation form EditCourseForm

We can't pass Structs when updating records in rails 8 any more, we need
to pass nil not Struct(day: 1, month: nil, year: nil) when creating or
updating records.

app/forms/support/edit_course_form.rb

@@ -80,8 +80,8 @@ def assign_attributes_to_course
       attributes = {
         course_code:,
         name:,
-        start_date:,
-        applications_open_from:,
+        start_date: valid_date?(:start_date) ? start_date : nil,
+        applications_open_from: valid_date?(:applications_open_from) ? applications_open_from : nil,
         is_send: send?,
         can_sponsor_student_visa:,
         can_sponsor_skilled_worker_visa: