style: if ! changed by unless

- if ! changed by unless as it is more readable
- !object.blank? is equivalent to object.present?, also more readable

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

@@ -106,13 +106,13 @@
           %td
             = @event.language
 
-      - if !@event.room.nil?
+      - unless @event.room.nil?
         %tr
           %td
             %b Room
           %td
             = @event.room.name
-      - if !@event.start_time.nil?
+      - unless @event.start_time.nil?
         %tr
           %td
             %b Scheduled time