master courses: avoid unnecessary child tag creation errors

use content_tag_for on creation so we can populate the
@content_tag_index for reuse in the migration

closes #CNVS-38017

Change-Id: Ia1b9fd773f55b4e3dcd068c740896ff0ec6d43e0
Reviewed-on: https://gerrit.instructure.com/122108
Tested-by: Jenkins
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>

app/models/master_courses/restrictor.rb

@@ -125,7 +125,7 @@ module MasterCourses::Restrictor
   def create_child_content_tag
     # i thought about making this a bulk insert at the end of the migration but the race conditions seemed scary
     if @importing_migration && is_child_content?
-      @importing_migration.master_course_subscription.create_content_tag_for!(self)
+      @importing_migration.master_course_subscription.content_tag_for(self)
     end
   end
 

app/models/master_courses/tag_helper.rb

@@ -46,6 +46,7 @@ module MasterCourses::TagHelper
     if content.is_a?(Assignment) && submittable = content.submittable_object
       content = submittable # use one child tag
     end
+    return unless content && content.persisted?
 
     if @content_tag_index
       tag = (@content_tag_index[content.class.base_class.name] || {})[content.id]