Do not validate blank default custom field value.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@8722 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/models/custom_field.rb

@@ -57,7 +57,7 @@ class CustomField < ActiveRecord::Base
       end
     end
 
-    unless valid_field_value?(default_value)
+    if default_value.present? && !valid_field_value?(default_value)
       errors.add(:default_value, :invalid)
     end
   end