Fix event registatrion for rooms without size

If the room size is not set (which is possible as the size is not
mandatory), the events registrations page breaks because of a
comparison of nil with Integer.

app/views/admin/events/registrations.html.haml

@@ -11,7 +11,7 @@
         for
         = @event.title
 
-    - if @event.room && (@event_registrations.length > @event.room.size)
+    - if @event.room.try(:size) && (@event_registrations.length > @event.room.size)
       %b Attention:
       You have more registrations than the capacity of the room!