Don't try to show assignee in approved_merge_request_email if there's no assignee

Signed-off-by: Rémy Coutable <remy@rymai.me>

app/views/notify/approved_merge_request_email.html.haml

@@ -121,6 +121,8 @@
                                           %td{ style: "font-family:'Helvetica Neue',Helvetica,Arial,sans-serif;font-size:15px;line-height:1.4;vertical-align:middle;" }
                                             %a.muted{ href: user_url(@merge_request.author), style: "color:#333333;text-decoration:none;" }
                                               = @merge_request.author.name
+
+                                - if @merge_request.assignee
                                   %tr
                                     %td{ style: "font-family:'Helvetica Neue',Helvetica,Arial,sans-serif;font-size:15px;line-height:1.4;color:#8c8c8c;font-weight:300;padding:14px 0;margin:0;border-top:1px solid #ededed;" } Assignee
                                     %td{ style: "font-family:'Helvetica Neue',Helvetica,Arial,sans-serif;font-size:15px;line-height:1.4;color:#8c8c8c;font-weight:300;padding:14px 0;margin:0;color:#333333;font-weight:400;width:75%;padding-left:5px;border-top:1px solid #ededed;" }

changelogs/unreleased-ee/1601-fix-approved_merge_request_email-when-no-assignee.yml

+---
+title: Don't try to show assignee in approved_merge_request_email if there's no assignee
+merge_request:
+author:

spec/mailers/notify_spec.rb

@@ -403,6 +403,16 @@ describe Notify do
             is_expected.to have_body_text /#{merge_request.assignee.name}/
             is_expected.to have_body_text /#{last_approver.name}/
           end
+
+          context 'when merge request has no assignee' do
+            before do
+              merge_request.update(assignee: nil)
+            end
+
+            it 'does not show the assignee' do
+              is_expected.not_to have_body_text 'Assignee'
+            end
+          end
         end
 
         describe 'that are unapproved' do