Fix group members method for project import/export

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

lib/gitlab/import_export/project_tree_saver.rb

@@ -47,7 +47,13 @@ module Gitlab
       def group_members
         return [] unless @current_user.can?(:admin_group, @project.group)
 
-        MembersFinder.new(@project.project_members, @project.group).execute(@current_user)
+        # We need `.where.not(user_id: nil)` here otherwise when a group has an
+        # invitee, it would make the following query return 0 rows since a NULL
+        # user_id would be present in the subquery
+        # See http://stackoverflow.com/questions/129077/not-in-clause-and-null-values
+        non_null_user_ids = @project.project_members.where.not(user_id: nil).select(:user_id)
+
+        GroupMembersFinder.new(@project.group).execute.where.not(user_id: non_null_user_ids)
       end
     end
   end