Optimization to not hit the db when preloading empty association sets

Optimized patch in preload_belongs_to_association method.

Here's the case where I discovered this behavior:

class User < ActiveRecord::Base
  has_many :memberships
  has_many :sites, :through => :memberships
  belongs_to :current_membership, :class_name => "Membership"
end

class Membership < ActiveRecord::Base
  belongs_to :user
  belongs_to :site
end

class Site < ActiveRecord::Base
  has_many :memberships
  has_many :users, :through => :memberships
end

which generates the following SQL:

  User Load (0.3ms)   SELECT * FROM `users` LIMIT 1
  Membership Columns (2.2ms)   SHOW FIELDS FROM `memberships`
  Membership Load (0.2ms)   SELECT * FROM `memberships` WHERE (`memberships`.`id` = NULL)

Oops. Forgot the find that generated the SQL:

@@@ User.first :include => :current_membership

Here's my take. assert_queries does the job nicely for testing this - there was an existing case that tested the previous behaviour for belongs_to. There is actually no need to do this for the has_many etc... cases, since we check earlier on that we have at least one record (belongs_to is different because you can have records but still have no associations to load if the foreign keys are all null)