eliminate duplicate code from to_sql

I don't really like passing the block, but this seems easiest for now

activerecord/lib/active_record/relation.rb

@@ -507,9 +507,7 @@ def to_sql
                     visitor    = connection.visitor
 
                     if eager_loading?
-                      join_dependency = construct_join_dependency
-                      relation        = except :select
-                      relation        = construct_relation_for_association_find(join_dependency, relation)
+                      find_with_associations { |rel| relation = rel }
                     end
 
                     ast   = relation.arel.ast

activerecord/lib/active_record/relation/finder_methods.rb

@@ -244,11 +244,15 @@ def find_with_associations
       join_dependency = construct_join_dependency
       relation = except :select
       relation = construct_relation_for_association_find(join_dependency, relation)
-      if ActiveRecord::NullRelation === relation
-        []
+      if block_given?
+        yield relation
       else
-        rows = connection.select_all(relation.arel, 'SQL', relation.bind_values.dup)
-        join_dependency.instantiate(rows)
+        if ActiveRecord::NullRelation === relation
+          []
+        else
+          rows = connection.select_all(relation.arel, 'SQL', relation.bind_values.dup)
+          join_dependency.instantiate(rows)
+        end
       end
     end