Fix logic error in MediaObject.ensure_attachment

closes LA-431
flag=none

test plan:
  - spec/models/media_object_spec.rb passes

Change-Id: I42b1138e9f6a89b2b7fda4b1749f70145c439c37
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/222360
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Ed Schiebel <eschiebel@instructure.com>

app/models/media_object.rb

@@ -298,7 +298,7 @@ class MediaObject < ActiveRecord::Base
   def ensure_attachment
     return if self.attachment_id
     sources = self.media_sources
-    return unless sources&.length
+    return unless sources.present?
 
     attachment = build_attachment({
       "context" => context,

spec/models/media_object_spec.rb

@@ -268,6 +268,14 @@ describe MediaObject do
       mo.retrieve_details
       expect(mo.media_type).to eql("")
     end
+
+    it "doesn't create the attachment until media_sources exist" do
+      allow(@mock_kaltura).to receive(:media_sources).and_return([])
+      mo = @media_object
+      mo.process_retrieved_details(@mock_entry, @media_type, @assets)
+      att = Attachment.where(:media_entry_id => mo[:media_id])
+      expect(att).to be_empty
+    end
   end
 
   describe ".guaranteed_title" do