fix(dashboard): fix ProjectSystemDefaultChangeException

[mingming-cn]

This PR is to fix a ProjectSystemDefaultChangeException that occurs when allowed_projects.count equals 1.

If you update the default project dashboard, the following exception will occur:

Dashboard::ProjectSystemDefaultChangeException (Dashboard::ProjectSystemDefaultChangeException):

plugins/additionals/app/models/dashboard.rb:389:in 'validate_project_system_default'
plugins/additionals/app/controllers/dashboards_controller.rb:113:in 'update'
lib/redmine/sudo_mode.rb:61:in 'sudo_mode'