Added check for image_tag occurences.

Checking for presence of a image before using image_tag
Fixes #1909

app/views/admin/sponsors/index.html.haml

@@ -19,7 +19,8 @@
           - @conference.sponsors.each do |sponsor|
             %tr
               %td
-                = image_tag(sponsor.picture.thumb.url, width: '20%')
+                - if sponsor.picture?
+                  = image_tag(sponsor.picture.thumb.url, width: '20%')
               %td
                 = sponsor.name
               %td

app/views/conferences/_modal_description.haml

@@ -15,7 +15,8 @@
                 - if object.is_a? Sponsor
                   - img_classes << ['img-sponsor',
                     "img-sponsor-#{object.sponsorship_level.position}"]
-                = image_tag get_logo(object), class: img_classes
+                - if object.picture?
+                  = image_tag get_logo(object), class: img_classes
             %p.description
               = object.description
           .modal-footer