Supports talks without speakers

Admins may want to have discussions or any other general activity
that does not have any particular speaker. Thus, speaker validation
is removed.

Closes https://github.com/openSUSE/osem/issues/2520

app/views/admin/reports/_events_without_commercials.html.haml

@@ -18,4 +18,8 @@
           %td= event.id
           %td= link_to event.title, edit_admin_conference_program_event_path(@conference.short_title, event)
           %td
-            = speaker_links(event)
+            - if(event.speakers.any?)
+              = speaker_links(event)
+            - else
+              .small
+                (No speaker)