fix bug where people would not see branding on dashboard

fixes: #CNVS-26358

test plan:

* log into canvas (so @current_user exists)

* go to the dashboard on a domain where you are
  not enrolled in anything (if you use pow this 
  could be something like blankaccount.canvas-lms.dev)

* you should see blankaccount's branding 

Change-Id: Ieef00e50eb6cc5c3d09d90441bd257ecf316af99
Reviewed-on: https://gerrit.instructure.com/70055
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins
QA-Review: August Thornton <august@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>

Author: ryankshaw

app/helpers/application_helper.rb

@@ -694,11 +694,11 @@ def brand_config_for_account(opts={})
         # if I'm at saltlakeschooldistrict.instructure.com, but I'm only enrolled in classes at
         # Highland High, show Highland's branding even on the dashboard.
         account = @current_user.common_account_chain(@domain_root_account).last
-      else
-        # If we're not logged in, and we're on the dashboard at
-        # saltlakeschooldistrict.instructure.com, just show it's branding
-        account = @domain_root_account
       end
+      # If we're not logged in, or we have no enrollments anywhere in domain_root_account,
+      # and we're on the dashboard at eg: saltlakeschooldistrict.instructure.com, just
+      # show its branding
+      account ||= @domain_root_account
     end
 
     if account && (brand_config = account.effective_brand_config)

spec/helpers/application_helper_spec.rb

@@ -566,6 +566,13 @@ def set_up_subaccounts
             expect(output.scan(%r{https://example.com/(root|child|grandchild)?/account.css})).to eql [['root'], ['child'], ['grandchild']]
           end
 
+          it "should fall-back to @domain_root_account's branding if I'm logged in but not enrolled in anything" do
+            @current_user = user
+            output = helper.include_account_css
+            expect(output).to have_tag 'link'
+            expect(output.scan(%r{https://example.com/(root|child|grandchild)?/account.css})).to eql [['root']]
+          end
+
           it "should include site admin css/js" do
             raise pending("need to handle site admin css/js from theme editor: CNVS-25229")
           end