add ":id => nil" to "Overall activity" link_to

On Rails 3.2.1, "Overall activity" link becomes "/assets?controller=activities".

Contributed by John Yani.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@8922 e93f8b46-1217-0410-a6f0-8f06a7374b81