Fix blank address which are proposed in new event form.

app/views/events/_form.html.haml

@@ -33,7 +33,7 @@
     = f.label :address
     = f.text_field :address, list: :addresses
     %datalist#addresses
-      - Event.group(:address).order('count(address) desc').pluck(:address).each do |address|
+      - Event.where('address is not null').group(:address).order('count(address) desc').pluck(:address).each do |address|
         %option= address
   .field.city
     = f.label :city