fix user quota calculation with no submission attachments

test plan:
* create a new user
* as the user, upload a new file to the user's folder
* refresh the page
* the quota should be updated

close #CNVS-27064

Change-Id: Ic16b807428802f24b2dd502de25bdbd4902f6d82
Reviewed-on: https://gerrit.instructure.com/71606
Tested-by: Jenkins
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>

app/models/attachment.rb

@@ -582,7 +582,7 @@ class Attachment < ActiveRecord::Base
 
           if context.is_a?(User)
             excluded_attachment_ids = context.attachments.joins(:attachment_associations).where("attachment_associations.context_type = ?", "Submission").pluck(:id)
-            attachment_scope = attachment_scope.where("id NOT IN (?)", excluded_attachment_ids)
+            attachment_scope = attachment_scope.where("id NOT IN (?)", excluded_attachment_ids) if excluded_attachment_ids.any?
           end
 
           min = self.minimum_size_for_quota

spec/models/attachment_spec.rb

@@ -1251,6 +1251,9 @@ describe Attachment do
       attachment_model(:context => @user, :uploaded_data => stub_png_data, :filename => "homework.png")
       @attachment.update_attribute(:size, 1.megabyte)
 
+      quota = Attachment.get_quota(@user)
+      expect(quota[:quota_used]).to eq 1.megabyte
+
       @assignment = @course.assignments.create!
       sub = @assignment.submit_homework(@user, attachments: [@attachment])