Prevent 500 on MR page if merge_event missing

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/views/projects/merge_requests/show/_state_widget.html.haml

@@ -19,8 +19,10 @@
 
     - if @merge_request.merged?
       %h4
-        Merged by #{link_to_member(@project, @merge_request.merge_event.author, avatar: false)}
-        #{time_ago_with_tooltip(@merge_request.merge_event.created_at)}
+        Merged
+        - if @merge_request.merge_event
+          by #{link_to_member(@project, @merge_request.merge_event.author, avatar: false)}
+          #{time_ago_with_tooltip(@merge_request.merge_event.created_at)}
       = render "projects/merge_requests/show/remove_source_branch"
 
     - if @merge_request.locked?
@@ -46,4 +48,3 @@
         Accepting this merge request will close #{@closes_issues.size == 1 ? 'issue' : 'issues'}
         = succeed '.' do
           != gfm(issues_sentence(@closes_issues))
-