fix listed_authors check

mkt/site/helpers.py

@@ -55,7 +55,7 @@ def product_as_dict(request, product):
     # Dev environments might not have authors set.
     author = ''
     author_url = ''
-    if product.listed_authors[0]:
+    if product.listed_authors:
         author = product.listed_authors[0].name
         author_url = product.listed_authors[0].get_url_path()
 
@@ -66,8 +66,8 @@ def product_as_dict(request, product):
         'preapprovalUrl': reverse('detail.purchase.preapproval',
                                   args=[product.app_slug]),
         'recordUrl': product.get_detail_url('record'),
-        'author': author
-        'author_url': author_url
+        'author': author,
+        'author_url': author_url,
         'icon_128_url': product.get_icon_url(128)
     }
     if product.is_premium():