Use render_if_exists for EE partials

To eliminate CE/EE difference

app/views/shared/milestones/_sidebar.html.haml

@@ -99,7 +99,7 @@
           = _('Time tracking')
           = icon('spinner spin')
 
-    = render 'shared/milestones/weight', milestone: milestone
+    = render_if_exists 'shared/milestones/weight', milestone: milestone
 
     .block.merge-requests
       .sidebar-collapsed-icon.has-tooltip{ title: milestone_merge_requests_tooltip_text(milestone), data: { container: 'body', html: 'true', placement: 'left', boundary: 'viewport' } }

app/views/shared/milestones/_top.html.haml

@@ -48,7 +48,7 @@
     - close_msg = group ? 'You may close the milestone now.' : 'Navigate to the project to close the milestone.'
     %span All issues for this milestone are closed. #{close_msg}
 
-= render 'shared/milestones/burndown', milestone: @milestone, project: @project
+= render_if_exists 'shared/milestones/burndown', milestone: @milestone, project: @project
 
 - if is_dynamic_milestone
   .table-holder