make count(:conditions => {}) work on has_many associations

When using conditions from a variable, it can sometimes be an empty hash (at least in my projects). When using it with count on a ActiveRecord::Base:

User.count :conditions => {}
# => 3

But when working on a has_many association, this happens:

>> User.first.certificates.count :conditions => {}
ActiveRecord::StatementInvalid: Mysql::Error: #42000You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '))' at line 1: SELECT count(*) AS count_all FROM `certificates`     WHERE (`certificates`.user_id = 442758120 AND ()) 

a patch to fix this has been attached