minor fixes for HasContentTags

refs CNVS-6947

 * don't look for them on folders
 * don't do updates if no tags were found

test plan:
 * from the UI, download a zip of a folder
 * check the log for the ContentZipper job that created the zip
 * there shouldn't be a query against content_tags

Change-Id: I51097956a2ee4d77f42f2019c6e563280af8f296
Reviewed-on: https://gerrit.instructure.com/22410
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>

Author: ccutrer

app/models/content_tag.rb

@@ -264,15 +264,15 @@ def available_for?(user, opts={})
 
   def self.update_for(asset)
     tags = ContentTag.where(:content_id => asset, :content_type => asset.class.to_s).not_deleted.select([:id, :tag_type, :content_type, :context_module_id]).all
-    module_ids = tags.select{|t| t.context_module_id }.map(&:context_module_id)
+    module_ids = tags.map(&:context_module_id).compact
 
     # update title
     tag_ids = tags.select{|t| t.sync_title_to_asset_title? }.map(&:id)
     attr_hash = {:updated_at => Time.now.utc}
     {:display_name => :title, :name => :title, :title => :title}.each do |attr, val|
       attr_hash[val] = asset.send(attr) if asset.respond_to?(attr)
     end
-    ContentTag.where(:id => tag_ids).update_all(attr_hash)
+    ContentTag.where(:id => tag_ids).update_all(attr_hash) unless tag_ids.empty?
 
     # update workflow_state
     tag_ids = tags.select{|t| t.sync_workflow_state_to_asset? }.map(&:id)
@@ -284,7 +284,7 @@ def self.update_for(asset)
         attr_hash[:workflow_state] = asset.workflow_state
       end
     end
-    ContentTag.where(:id => tag_ids).update_all(attr_hash) if attr_hash[:workflow_state]
+    ContentTag.where(:id => tag_ids).update_all(attr_hash) if attr_hash[:workflow_state] && !tag_ids.empty?
 
     ContentTag.touch_context_modules(module_ids)
   end

lib/has_content_tags.rb

@@ -29,7 +29,7 @@ def update_associated_content_tags
   def check_if_associated_content_tags_need_updating
     @associated_content_tags_need_updating = false
     return if self.new_record?
-    return if self.respond_to?(:context_type) && self.context_type == 'SisBatch'
+    return if self.respond_to?(:context_type) && %w{SisBatch Folder}.include?(self.context_type)
     @associated_content_tags_need_updating = true if self.respond_to?(:title_changed?) && self.title_changed?
     @associated_content_tags_need_updating = true if self.respond_to?(:name_changed?) && self.name_changed?
     @associated_content_tags_need_updating = true if self.respond_to?(:display_name_changed?) && self.display_name_changed?