allow create brand config without brand_config

fixes CORE-856

test plan
 - upload a file to a brand config
 - it should work

Change-Id: I3b03ffe1aceded1fcfa9fcaf83b606293b2a4789
Reviewed-on: https://gerrit.instructure.com/138948
Reviewed-by: James Williams  <jamesw@instructure.com>
Tested-by: Jenkins
Product-Review: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>

app/controllers/brand_configs_controller.rb

@@ -104,6 +104,7 @@ class BrandConfigsController < ApplicationController
   # indicating the progress of generating the css and pushing it to the CDN
   # @returns {BrandConfig, Progress}
   def create
+    params[:brand_config] ||= {}
     opts = {
       parent_md5: @account.first_parent_brand_config.try(:md5),
       variables: process_variables(params[:brand_config][:variables])
@@ -188,6 +189,7 @@ class BrandConfigsController < ApplicationController
   end
 
   def process_variables(variables)
+    return unless variables
     variables.to_unsafe_h.each_with_object({}) do |(key, value), memo|
       next unless value.present? && (config = BrandableCSS.variables_map[key])
       value = process_file(value) if config['type'] == 'image'

spec/controllers/brand_configs_controller_spec.rb

@@ -108,6 +108,12 @@ describe BrandConfigsController do
       expect(json['brand_config']['variables']['ic-brand-primary']).to eq "#000000"
     end
 
+    it "should not fail when a brand_config is not passed" do
+      user_session(admin)
+      post 'create', params: {account_id: @account.id}
+      assert_status(200)
+    end
+
     it 'should not allow non admin access' do
       user = user_with_pseudonym(active_all: true)
       user_session(user)