No. It is not necessary and some people don’t use it at all or only use it during the winter when the air is very dry due to heating. The purpose of the humidifier is to moisten the incoming air so that it doesn’t dry the sinuses. If you breathe solely through your nose, your nasal passages naturally humidify the incoming air.
Your cpap machine should not stop producing airflow at any time. If it is an auto adjusting machine, it might reduce your pressure during the night, but it should not completely stop producing airflow. Contact your CPAP equipment provider and make arrangements to have your CPAP machine airflow problems checked.