Force TimeEntry#hours default to nil (#3075, #4449).

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@3232 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/models/time_entry.rb

@@ -44,6 +44,7 @@ class TimeEntry < ActiveRecord::Base
       if default_activity = TimeEntryActivity.default
         self.activity_id = default_activity.id
       end
+      self.hours = nil if hours == 0
     end
   end