Do not call link_to_reportable if reportable does not exist

We call the `link_to_reportables` helper on the report show view.
This throws an `nil` exception when the reportable is deleted,
since the `reportable_type` gets nullified as well.
`link_to_reportable` works in on the email notification since the
`reportable_type` is still available from the event payload, but
we cannot retrieve it from the report record.

Fixes openSUSE#16062

src/api/app/views/webui/reports/show.html.haml

@@ -8,8 +8,9 @@
     %p
       Created by
       = render UserAvatarComponent.new(@report.user.login)
-      on
-      = link_to_reportables(report_id: @report.id, reportable_type: @report.reportable_type, host: '')
+      - if @report.reportable.present?
+        on
+        = link_to_reportables(report_id: @report.id, reportable_type: @report.reportable_type, host: '')
     %h5 Description
     %p
       = @report.reason